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joint. When it is desired to couple up a pipe of smaller size, a “bushing" is used; this screws over the small pipe with its internal threading and into the large pipe by its external threading. Two parallel pipes may be connected by a "fitting" called a return bend, made semicircular; or pipes may be joined by a Y-branch: this is where two pipes are rooted into one. The fitting called an elbow is to conduct a pipe in an angle of 90° or a right angle.

[graphic]

FIG. 449A.-MODERN STORE ROOM-FINISHED WORK.

APPENDIX

Shaw's Electrical Measuring Machine. The foregoing measuring machines depend on the principle of compression, i.e. a feeler or "gravity piece" is directly or indirectly gripped between the head and the gauge; when the head is made to recede the "gravity piece" falls, and the reading on the machine is then taken.

But another principle, that of electric touch, has recently been proved by Dr. Shaw, Nottingham, to be much more sensitive than that

FIG 450.-Shaw's electrical measuring machine.

of compression. When the head touches the gauge an electric circuit is completed through a telephone carried by the observer. Instead of using heads which are plain or slightly convex, as in the compression machines, one or both heads are rounded points or beads. The screw has a pitch of in., the wheel has 500 divisions, and the vernier shows tenths-so the unit is 250000 in. It is as easy to read in. on a compression machine.

a difference of this amount as

25000

This great gain in sensitiveness is not due to any improvement in the mechanism, but to the totally different principle of detecting touch.

In the form of instrument shown there is a fixed base which is rendered optically plain; on this the gauge is placed, and the screw is lowered until electric contact is made. The electrical accessories are simple and easily arranged.

Diameter.

WHITWORTH STANDARD SCREW THREADS. (FIG. 451.)

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Diameter.

No. of threads per

No. o

Diameter.

inch.

per

threads

inch.

Diameter.

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