COMMERCIAL RATING OF ELECTRIC

GENERATORS.*

BY THOMAS G GRIER.

GOOD engineering requires that specifications should have their aims, objects, and purposes defined with accuracy. The generator of an electric light or power plant is one of its most essential features, and in defining or in rating its capacity we should do so in terms that have a tangible meaning.

The standard electrical units by which electric light installations may be specified are the ohm, volt, ampère and candlepower, and in electrical power installations substituting the horse-power for the candle-power.

Instruments for measuring the ohm, volt, and ampère have been adapted to the necessities of practical engineering, and when these three units are the basis of a specification it is with comparative ease that an expert can ascertain the correctness of the work.

In the majority of specifications and contracts, as now drawn, the capacity of the wiring will be for so many 16 candle-power lamps, or for so many horse-power motors; and the generators will be rated in so many 16 candle-power lamp capacity, or for so many horse-power motors.

This use of the units, candle-power, and horse-power, virtually means nothing.

The arc lamp of so many nominal candle-power is another unit which means nothing when applied to the capacity of the machine.

The candle power unit is in a measure open for criticism on the point of accuracy, but in the discussion the discrepancies that may exist in the several standards are not the points to be considered. The candle-power unit is a unit of light. Our generators do not produce light, but electrical energy. Our arc and incandescent lamps convert the electrical energy into heat and light. The horse-power is the unit of power. Our generators do not produce the power, but our motors utilise the electrical energy and convert it.

Before considering why we should not use the terms candle-power and horse-power in rating our generators, even though these terms are perfectly correct, we will discuss the converting apparatus, that is, the incandescent lamp, the arc lamp and the motor.

The incandescent lamp being an apparatus for converting electrical energy into light with an unfortunate side product of heat, does not give back 100 per cent. of the electrical energy as light, nor does the proportion that produces light and the proportion that develops heat vary in the same ratio under different conditions. Lamps are in use to-day that vary in the consumption of energy per candle-power produced from 3 to 7 watts, and even as low as 2 watts has been claimed. This variation in the energy, when the lamps are of the same voltage, means a variation in the amount of current necessary to produce the equal amount of light. Recent experiments made upon lamps bought in the open market show conclusively that lamps of the same rated voltage and candle-power are not uniform. Here are a few instances of the variations of the amount of current required.

First, 20 lamps, 10 each of two different makes, were placed upon ordinary commercial meters, and were burned for the same length of time. To eliminate the errors of the meters they were transposed so that each set burned an equal time on each meter. The result of the experiment showed that there was a difference of 27 per cent., in the amount of current consumed, between the sets.

2. Another station tried the same experiment, using one set of lamps of a different make from that of the former, and one set of the same make. The result of their experiment was a difference of 22 per cent.

3. A station operating some 700 lights had a number of the nominal 16 candle-power lamps that were in use tested, and found the candle-power to range from 17 to 22 candlepower. On discovering this, nominal 10 candle-power lamps were used and called 16 candle-power. As the current was

Read before the Chicago Electric Club, January 19th, 1891.

sold by meter, the customer was in no way injured by the substitution.

4. A number of large lamps were tested, and nominal 100 candle-power lamps were found to give from 44 candle-power up to 110 candle-power.

5. In December, 1890, tests were made on four different makes of lamps. All the lamps were marked by the manufacturers at the same voltage, and all were nominal 16 candlepower lamps. The tests showed a variety of candle-powers ranging from 11 up to 24, and efficiencies from 2:46 watts to 4 watts per candle-power.

6. A large lamp of new make of nominal 150 candle-power gave an efficiency of 24 watts per candle-power. As this was the most efficient of any lamp tested it has been placed on the circuit of a central station, and is undergoing a life

test.

These experiments are of considerable value in this paper as proofs of the desired end in view, as they show that when capacity is given in candle-power it means only an approximation. We, however, should not draw conclusions as to the real commercial value of the lamps tested.

The durability of a lamp is a most important factor, and without a test as to the life of the various lamps and a mathematical equation being deduced that would include life, efficiency, and cost of lamp, no actual commercial conclusion should be drawn. It is a well known fact that the life of a lamp is dependent upon the efficiency at which it is used. A lamp can be made to produce light commercially at an expenditure of energy varying from 2 to 7 watts per candle

power.

The factors that govern the efficiency at which the lamp should be run are, the cost of producing the electrical energy, and the cost of the lamps.

From this view of the matter, even though the lamps were uniform, it is possible to have conditions wherein would exist a difference of over 100 per cent. in the amount of current necessary to produce equal amounts of light.

When a specification requires that an incandescent dynamo should have a capacity of 100 16-candle-power lamps at 110 volts, what does it mean? Must the machine be capable of producing energy sufficient to operate lamps at 7 watts, or but 2 watts per candle-power.

Must it develop 30 ampères, or about 102 ampères ? Either machine being within the range of the specification, the efficiency of the lamps entirely governing the size. If nothing further than the capacity in candle-power is specified what right would the purchaser have to grievance or redress if the machine of the smallest limit (that is 30 ampères) were sold to him as a 100-light machine, his lamps requiring more ampères than 30?

The arc lamp is also an apparatus which converts our electrical energy into light and heat. Its efficiency when well adjusted and carefully cared for, should not vary to any great extent. Bad adjustment is a baneful cause of poor efficiency, and good adjustment cannot be maintained without good regulation. To measure the candle-power of an arc lamp is not the easiest matter in the world, and for one to attempt to judge by the eye is an absurdity.

In tests that have been made on arc lamps, the results were fromths of a horse-power up to 1 ths horse-power per lamp of 2,000 candle-power, so-called. These figures were based upon the difference between the total horse-power exerted by the engine, and the horse-power of the engine running empty and the belt thrown off the dynamo.

As the actual candle-power of the lamp could not be measured it remains still a matter of conjecture whether the machines were really of so many 2,000 candle-power lamp capacity.

The motor is another apparatus for transforming electrical energy into different forms. The efficiencies of the various motors on the market vary from 90 odd per cent. to as low as 50 per cent., and perhaps lower. Their efficiency also varies with their speed and load.

The generator for a 10 horse-power motor could mean a machine capable of generating anywhere from 11 horsepower up to 20, depending entirely upon the economy with which the electric motor would utilise the energy supplied by the generator.

The question of rating the machines in the units which measure the work of another piece of apparatus is merely rating the machine upon a variable standard, that is, the

efficiency of another machine. We do not rate our engines in the capacity of the dynamo, but upon a unit of its ownthe horse-power, giving the speed and pressure at which this horse-power is developed.

The boilers, however, are frequently rated in the unit horse-power, which unfortunately is as great a mistake as the rating of dynamos or generators in any other unit than that of the form of energy which is directly produced by them.

The transformation, or the conversion, of any form of energy to another must take into consideration the efficiency of the converting apparatus.

In transmitting energy from the point of production to the point of consumption, a loss is entailed. To convey electrical energy for light and power we use a metal conductor. The size of this conductor is not dependent upon the amount of horse-power developed or the amount of candle-power developed, but upon the amount of current transmitted, and the amount of pressure behind it. It is within the limits of possibilities wherein the current to develop the same amount of light or power may be double in one case what it would in another, owing to the difference in efficiency of the converting apparatus.

Another point to consider besides the efficiency is the resistance in the conductors. In some cases where fuel is extremely cheap, a large percentage of the energy may be used in transmitting the power, and consequently the generator would have to have an increase added to its capacity in proportion to this percentage that is lost. Tables for the use of wiremen have been prepared by the various companies, and while speaking of the generator it is just as well that attention should be called to the tables. Units used as a basis for their determination are the horse-power and the 16 candlepower lamp. It is apparent that a wire to carry a 16 candlepower lamp or a horse-power with a certain per cent. loss, does not even mean an approximation, the variable conditions that exist giving the expression only a very general meaning. The lamps or horse-power are assumed to take a certain amount of current, and, though the calculations are correct, the entire table depends for its accuracy upon how near the assumption approaches the actual efficiency of the apparatus used.

The capacity of the generators should be specified in the units that measure the energy in the form in which it is developed. Dynamo-electric machines develop or produce electrical energy measured in volts and ampères. The carrying capacity of wires for the transmission of this energy depends upon volts and ampères. The candle-power and horse-power have naught to do with it. Electrical generators should have their capacity specified in volts and ampères or in watts giving the pressure in volts at which this specified power is produced. The size of the wires should be determined upon the basis of ampères and volts, stating the percentage or number of volts lost in transmitting the current.

The English manufacturers give the capacity of the dynamo in watts, and a purchaser, even if he be ignorant of the first principles of electric machinery, could with little trouble find whether he had received the capacity that he had paid for. But had the rating been in candle-power or horsepower the result could not have been so satisfactory.

The Edison Company in the United States and several others of the larger electric manufacturing companies have given the capacity of their various generators in watts, but the agents and contractors seem invariably to overlook this, and propositions and contracts are drawn up with the generators rated in the indefinite terms, horse-power and lamps or candle-power.

Why the Americans have adopted the candle-power and the horse-power as a basis of rating electrical machinery and the compiling of wiring tables, is, perhaps, apparent to many; and the reasons were, no doubt, at one time good. It is, however, obvious that this general use of these technical terms is entirely wrong, and the electrician must be the means of reorganising the system. The tendencies are in the direction of this needed reformation, but the change cannot be too rapid nor too soon.

The fundamental principles of Ohm's law are thoroughly familiar to the intelligent workmen, and the sooner our calculations are confined to its mathematical accuracy, and our generating machinery rated in the proper units, the better for the electrical business.

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REVIEW.

[FEBRUARY 13, 1891.

AMERICAN INSTITUTE OF ELECTRICAL ENGINEERS.

AT the fifty-third meeting of the Institute, held on January 20th, the evening was devoted to the thorough discussion of Mr. Kennelly's paper on "Inductance and its Proposed Unit, the Henry," and the report of the committee appointed to consider the proper magnitude of the unit. The report of the committee was finally adopted, and is as follows:

"At a meeting of the American Institute of Electrical Engineers, held September 16th, 1890, a committee consisting of Dr. Wm. E. Geyer, Dr. Louis Bell, George M. Phelps, Edward A. Colby, and Francis B. Crocker, chairman, was appointed to consider and report upon the best value for the Henry'-the proposed practical unit of self-induction or inductance.

"After careful consideration the committee has reached a conclusion and now recommends to the Institute the retention of the present value of the practical unit, which is 102 absolute units of inductance, usually expressed as a length equal to one earth's quadrant or 1,000,000,000 centimetres.

"The committee finds that this value is too high in some cases for convenience, as many practical measurements of inductance would have to be expressed in fractions of a Henry. At the same time other practical values would often amount to a number of Henrys, and sometimes to hundreds and even thousands of Henrys. It is, of course, impossible to have a unit fit all cases, and in this instance it happens that the average practical inductance would approximate one Henry in value. This may be considered fortunate. Small values may be conveniently expressed in millihenrys, analogous to milliampères, &c.

"The committee is of the opinion that even if the value of this unit (109 absolute units) had been much less convenient than it is, nevertheless it would probably be very unwise to change it, for the reason that it is derived from and related to the other practical units in the ratio of one to one, just as one ampère is equal to one volt divided by one ohm. To depart from this plan, which has always been followed heretofore, would tend to destroy the harmony of the entire system of practical electrical units. The same fact made it necessary to define the farad as a capacity of one coulomb at one volt, although this makes its value so enormous that the microfarad is always used in practical work. It appears, therefore, to the committee that the value of the Henry' should be fixed at 10° absolute units of inductance."

6

THE LONG CREDIT SYSTEM.
[COMMUNICATED.]

Ar the back of many a liquor bar out West, standing out in relief to the rows of bottles and glasses on the shelves, may be seen, if not a thing of beauty, at least the only one there that could claim any kindred with art, the illustration of a prone hound under which is inscribed the touching words, Poor Trust is Dead: Bad Pay Killed Him."

Although the designer be lost to fame, yet are his words words of wisdom, and oft has his homely sketch 'monished the thriftless one that for him could be no more chalkings up behind the door, and taught him the lesson that if he would have goods there he must pay for them, and if he could not pay he must go without.

It would not be amiss for the electrical trade if some of its clever members could devise such a simple means of appealing to the consciences of its customers and to its own. "Monthly account" may be read in many quotations, and is theoretically the custom of the trade; but there is more breach of this custom than observance. One cannot help thinking that it would be very much better for the industry if the go-as-you-please system, or want of system, were now made to give way to stricter rules.

Electrical business has been subjected to such rapid expansion, that what has been sufficient capital one year has been inadequate the next, and with extra money locked up in plant and book debts it has been hard work to keep up regular payments. Much of the supply has been taken by the rich, men who have their half-yearly audits or Christmas

FEBRUARY 13, 1891.]

In some of the larger companies so much has had to go to the lawyer, the expert, and the patent agent, that the manufacturer has often had to wait; dole being given only to the most hungry and persistent-the sturdy beggars who would not be denied-pending the raising of fresh capital. It is pleasing to say the state of the companies in this respect is much better than it was, but they have not always set a very good example. Dwindling business, as well as increasing, has been one cause of bad pay in some quarters; but, on the whole, the failures have not been so very many in proportion to the number of firms engaged; quite enough, however, to teach caution, and the lesson that every day a debt gets older it becomes less easy to collect, and may go fast and faster to the bad.

"Finance is the art of using other people's money," and some firms have doubtless used more of other people's than of their own, and and thought it clever to do so. We all laugh with the man, who deeply in debt, when asked by his friends "How can you sleep?" replied, "Oh! I can sleep all right; but I often wonder how my creditors can." We laugh, but we do not like him. The cold-blooded selfishness of keeping people without their money just because it suits you, and not because it is a case of dire necessity, is apparent to most.

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"Money answereth all things "-collectors especially. It also saves unpleasant questions, such as, "When shall I call again?" "Well! when will you have a board meeting?" "Is the secretary likely to be ill long?" "Hasn't the cashier come back from his holiday yet?" "If your Mr. William is not in just now, when do you think he will be in ?"&c. Its possession makes respectability. Was not he held respectable whose father kept a gig? Dires equûm, dires pictae restis et auri, the nag, the rug, and the gold implied by their possession.

There are a few firms of whom we hear it said, "The most respectable people in the trade: first-rate pay." It would be a good thing if the same could be said of all, which the remark itself shows that it cannot be.

The electrical business is a growing one, and comparatively safe. Numbers are living by it already, and yet more can find in it occupation and profit. At a time when so much money is on deposit at banks at next to no interest, waiting sound investment, and suspicious of the channels offering, it is remarkable that young firms of good character should not have more offers of support than needful, and that there should be no rush for well-devised lighting companies. There was a time when young men of integrity found no difficulty in borrowing money to commence and carry on business, but that was before the days of monster joint stock companies and large foreign loans at high interest. People with spare means seem nowadays more ready to risk their money in speculative stocks, or buy the better class ones at a high figure, than to look out for the employment of their wealth in rising genuine business. It is easier to go to a broker, take his advice and sign a transfer, than to investigate such matters, it is true; but in the end the trouble taken would be justified. The risk would not be greater, and the profit probably more. It is a consummation devoutly to be wished, were it only that payments might be made more regularly. Strict payment would doubtless weed out some weak ones, but this would not be an unmixed evil. It would make people look ahead, and generally keep them up to the mark, if it were understood that pay days must be observed solemnly. It would be better for them individually and generally.

Too often is heard the plea: "My customer has not paid me yet; when he does, I will pay you." This, although it may certainly be a reason for a deficiency in the wherewithal, is an excuse which does not excuse from the obligation to meet an account when due. Prices are based on various considerations, and one of them is interest on outlay and allowance for proportion of possible bad debts. Prompt payment all round would mean larger turnover for the capital employed and safer business, resulting in cheaper work and goods. The

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cheaper, the wider will be the range of business to be done and the more business the better for the trade. The third-class passenger is not an unremunerative customer of the railways, and the sooner electric light, traction as well as domestic and other power-perhaps even heating-become within the reach of the many, the better will it be for everyone connected with electricity.

Small profits and quick returns should be the order of the day. The profits are, I think, not often exorbitant. "Catch a flat," may be good advice, but they don't rise to every cast, they are not always on the feed, want a lot of play, and by their very nature are apt to struggle at the last minute and not get landed. The bulk of business done is at fair rates, and some cut too fine; but money does not always come in as quickly as it should. The best way is to make definite arrangements first. "Leave it to you, Sir," may pay the cabby, but not the electrical engineer or manufac turer. It is always a delicate matter to say to a man, "I wont supply you.' Competition is so rife that his reply would be, "All right, don't, I'll go to someone else." It is probably best, however, under such circumstances, to let someone else have the chance of the profit or bad debt.

I have gone into this subject at some length because I thought it could do no harm and might perhaps do some good, and this must be my apology to any of my readers who may feel inclined to charge me, as regards the length of this article, with longwindedness.

ABSTRACTS

OF PUBLISHED SPECIFICATIONS, 1889.

17,425. "Improvements in electric arc lamps." H. HARPER. Dated Nov. 2. 8d. Consists, first, in improved means for regulating the feed of the carbons, secondly, in an improved method of constructing the solenoid-core whereby it is rendered specially adapted for use with alternating currents, and, thirdly, to certain devices for enabling the carbons to be readily fixed and accurately centred and otherwise for ensuring efficiency in the working of the lamp. 5 claims. 20,244. Improvements in electric-generating machinery for use in connection with the electric lighting of railway trains." J. H. HOLMES. Dated Dec. 17. 11d. The inventor constructs a dynamo electric machine with two armatures arranged to revolve together but in separate magnetic fields. The one is the main generating armature, the other being for regulating the strength of the magnetic field of the generating armature. He separately excites the magnets of the machine from some external source such as a set of accumulators and provides the magnets ot the generating armature with two distinct exciting circuits. One circuit is a high resistance shunt circuit, and the other is of less resistance and is coupled up to the source of current so as to have the small regulating armature in series with it. The regulating armature is so coupled up that its own E.M.F. opposes the E.M.F. of the source of supply. 6 claims.

20,955. Improvements in dynano-electric machines, in part applicable to electro-motors." J. J. WOOD. Dated December 31st. 1s. 5d. Consists in part of improvements in automatic regulators for governing dynamo machines, in order that they shall generate uniform currents under all circumstances. Another part of the invention consists of improvements in the armatures of dynamos, the improvements pertaining chiefly to the armature core and to the supporting spider. 17 claims.

20,991. "Improvements in electric and magneto bells and apparatus to be used therewith." H. P. F. JENSEN and J. JENSEN. Dated December 31st. 8d. The inventors form the armature of bells or indicators of a thin piece of iron or steel or both suitably united, the upper end being made quite thin to form a spring and bent at right angles and having an eye for passing the core shank through it and through the top of the bell, one nut at the end of the shank serving to hold the armature, the break spring, the insulating disc, the core and the bell together. The bottom end of core is cut off square or sloping, and the lower end of the armature and of the break spring are bent and continued down below it, thus rendering access to the break spring easier than heretofore. 10 claims.

1890.

1,347. "An improved fluid for primary batteries." T. COAD. Dated January 25th. 4d. In 10 ounces of nitric acid is dissolved 4 ounces of mercury, then add 1 ounce of carbonate of potash, then 1 ounce of carbonate of soda, then 4 ounces of bichromate of potash, then 50 ounces of water, then 20 ounces of sulphuric acid, and then 12 ounces of common soda. When the above has become cold, to every 10 ounces of the fluid add 20 more ounces of water, 6 ounces of nitric acid and 4 ounces of common soda. 5 claims.

4,660. Improvements in devices for heating and cooking by means of electricity, including electric flat irons and other articles." H. R. BUTTERFIELD and W. MITCHELL. Dated March 25th. 8d. Consists in enclosing a layer of continuous wire arranged in coils, between two layers of asbestos, and give to the material thus produced the

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shape of an oven, a broiler, or other cooking utensil, article or structure, the ends of the coil being also protected by asbestos or other insulating material and the wire connecting terminally with circuit wires so that a current of electricity will pass through it. 16 claims.

5,474. Improvements in regulating and controlling the resistance of electric circuits." H. LYON and H J. LESLIE. Dated April 10th. 8d. Relates to resistance cells. Each cell consists of a pot or jar, preferably of circular shape and made of earthenware or other suitable material. In the bottom of this pot or jar is placed a plate of lead, or other suitable metal, in the form of an inverted cone or its equivalent. To this plate is connected one of the terminal wires passing either through a suitable hole at or near to the bottom of the pot or jar, or up to the top of the said pot or jar. On the top of the pot or jar is a suitable bar having a central hole in which is a metallic ferrule or bush through which passes freely a spindle carrying on its lower end a metallic plate in the form of an inverted cone, or its equivalent, to correspond with the plate in the bottom of the pot or jar, and on its upper end a small hand wheel or its equivalent. On this spindle is a cut a screw thread with which the end of a stud pin threaded in the ferrule or bush engages, thus forming a nut of the said ferrule or bush, and at the same time affording a ready means of allowing the spindle to move freely up and down in the said ferrule or bush when desired. The other terminal wire is connected to this ferrule or bush and thus indirectly to the spindle by means of small springs fixed to the said ferrule or bush, and adapted to be in constant contact with the spindle. The pot or jar is partially filled with water either with or without sulphuric acid, and by raising or lowering the upper coned plate the resistance is increased or diminished as desired By making the plates connected to the terminals of an inverted cone shape the gases generated between the plates is assisted to escape, so that there is no danger of an explosion within the cell when leaving the highest point of resistance. 3 claims.

6,176. "Improvements in secondary batteries." G. BARKER. (Communicated from abroad by H. Woodward, of Toronto.) Dated April 23rd. 8d. Consists essentially in forming each electrode of a series of cylindrical or otherwise shaped perforated tubes made of vulcanised rubber or other acid proof non-conducting material, the said tubes being passed through one or more holders or separators made of vulcanised rubber or other acid proof non-conducting material by which they are held at a fixed distance apart. Each tube contains a spindle or stem surrounded with lead filings, shavings or other small pieces of lead or alloy of lead, the upper ends of each spindle being attached to a horizontal lead connector, and two or more of the electrodes so constructed are placed in a cell preferably made of glass and containing the ordinary solution of sulphuric acid. 2 claims.

6,236. "A new or improved storage battery." A. J. JARMAN® Dated April 24th. 8d. Relates to an improved construction of storage battery, termed a "tertiary battery," in contradistinction to those storage batteries hitherto styled secondary batteries. The term "tertiary" is derived from the three distinct and separate actions which take place within the cell, viz.:-1. Decomposition of the water forming the electrolyte, or the major part thereof, into its constituent parts by electrolysis, namely, hydrogen and oxygen. 2. The storage of hydrogen and oxygen in the metal plates and their component parts by the alloying of the hydrogen with one of the metal plates (this gas being considered as a metal in a gaseous state), and the storage of oxygen in the other plate. 3. The generation or production of a current of electricity by the reunion or recombination of the hydrogen and oxygen stored in the metal plates.

66

R.

6,878. An improved electric-alarm device for clocks." SCHMITZ. Dated May 3rd. 8d. Claims-1. The combination, with a clock movement, of a shaft driven by said clock movement, a series of cam wheels on said shaft, a series of electric circuit closers operated by said cam wheels, switch plates connected mith the circuit closers, and electric alarm bells connected with the switch plates, substantially as set forth. 2. The combination, with a clock movement, of a shaft driven by said movement, a series of cams in different positions on said shaft, a series of circuit closers operated by the cams, switch plates connected with the circuit closers, and electric bells connected with the switch plates, substantially as set forth. 3. The combination, with a clock movement, of a shaft driven by said movement, a series of cams on said shaft, a series of circuit closers operated by the cams, switch plates connected with the circuit closers, electric bells connected with the switch plates, a hand on one end of said shaft, and a dial over which the hand can rotate, substantially as set forth. 4. The combination, with a clock movement, of a shaft driven by said movement, a series of cams mounted on said shaft, a series of pivoted levers, of which each has one end rested on the cam, a push rod on each lever, contact springs on which the push rods can act, switch plates connected with the contact springs, and electric alarm bells connected with the switch plates, substantially as set forth.

8,578. "Improvements in electric accumulators or secondary batteries in the manufacture and production of plates or elements therefor." J. Y. JOHNSON. (Communicated from abroad by C. Hering, of Paris.) Dated June 3rd. 8d. Relates chiefly to a method of preparation consisting essentially in employing a soluble salt of lead as the binding or cementing agent of the pulverulent oxides.

11 claims.

8,599. "Improvements in apparatus for the generation, distribution and commutation of electric currents." F. ANDERSON. Dated June 3rd. 8d. Relates generally to the procuring in two electric circuits, one of which is auxiliary to the other, of two alternating electric currents, with any required difference of phase of undulation of each current relatively to the other. It comprises the employment of one of these currents of a certain phase to actuate a commuting apparatus in, and at the periods of time proper to convert the alternating current of differentiated phase into a continuous or direct current. claims.

5

[FEBRUARY 13, 1891.

8,800. "Improvements in or relating to electric accumulators." W. P. THOMPSON. (A communication from abroad by I. Peral, of Spain.) Dated June 7th 8d. Each plate is composed of an outer rectangular frame of the metal and has a series of stout transverse ribs with a long notch or groove along each side. These grooves (after the plate has been removed from the mould in which it has been cast are opened out by any suitable device, so as to form recesses in which the paste may be securely held. Smaller ribs connect the cross ribs at right angles, and, if desired, these are in turn connected by parallel ones. 4 claims.

10,421. "An improved method or means for the prevention of accidents from the overheating of electrical wires." J. BRADFORD Dated July 5th. 6d. The inventor insulates wires in the usual way and encloses them in ordinary metal piping of small diameter. This pipe he again encloses within another of larger diameter, supporting it in the centre by means of perforated discs of suitable insulating material. The larger external pipe he puts in direct communication with the water supply cistern of the house, from which it is filled. 1 claim.

On page 172 of your present issue is a mis-statement which, if it only concerned myself I should not waste a pen-stroke in noticing. But the statement has been made that the distinguished physicists who composed the British Association Unit Committee (which sat in 1862-3-4) were guilty of applying a law that is only true of steady currents (Ohm's law) to currents that are not steady. This egregious dictum is certainly not supported by the committee's own report, as may be seen from page 102 of Fleeming Jenkin's edition of the collected reports, where the differential equations of the variable currents are given, and duly integrated. What Lord Rayleigh did about 20 years later was to point out that the approximations used in calculating the coefficients were not close enough, and left a small residual error.

The amusing circumstance is that this mis-statement should occur in a lecture on mental capacity (or its absence) by the one true infallible prophet who has for twelve months' past been busy in setting us all right, from the President of the Royal Society down to Your humble servant, Silvanus P. Thompson.

February 7th, 1891.

Cardew's Earthing Device.

I have been looking up descriptions of the Cardew earthing device, and can find none which really explain the working of this apparatus. Would you be so good as to elucidate the following point for me :

If the primary leaks to the secondary in a transformer, and there is a dangerous P.D. on the secondary mains, the secondary will be automatically earthed and the primary fuses will go.

Now, the point I wish to ascertain is, whether the current which blows the fuses on earthing is caused by the difference of potential between the earth, and the point on the secondary mains to which the earth wire is attached.

N. S.

[The action of the Cardew earthing device is simply this: It automatically connects the secondary circuit to earth in the event of the difference of potential between earth and the secondary ever rising to 400 volts from any cause. It then depends upon the insulation of the primin mains as to whether the main fuses are blown. In rup re it has been found on the London Company's sun by t whenever the primary and the secondary came into Christr,