176 BRIDGMAN. 



pulling on the upper end of the lever through a link motion L. 

 The nichrome measuring wire C is attached to the upper end of 

 the lever, and slides over a contact at D which is attached to a 

 plate insulated from the rest of the apparatus with a thin mica 

 washer. At E is the potential terminal fixed to the wire. Flexible 

 leads pass from E and D to the two potential terminals of the three 

 terminal plug, and the current terminal is attached at F. The second 

 current connection is simply the case, to which everything is grounded. 

 The details of connecting the lever apparatus to the three terminal 

 plug so that the specimen attached to the plug can be screwed into 

 the pressure cylinder as one self-contained piece offer no difficulties 

 of design, and need not be shown. 



The apparatus as shown is flexible enough to be adapted to a wide 

 variety of sorts of specimens. Variations in the diameter of the speci- 

 men may be allowed for either by making different steel caps, or 

 preferably simply by making suitable steel collars to slip around the 

 specimen by which it is held securely in the pointed end pieces. Varia- 

 tions in the length of the specimen are allowed for by changing the 

 length of the dummy piece of iron at H. Specimens used with the 

 apparatus as shown must be stiff enough to support the compressive 

 stress due to the spring on the lever. But the compressibility of 

 wires may also be measured by a simple change that will readily sug- 

 gest itself, replacing the specimen S by a slotted frame of iron in which 

 is laid the specimen in the form of wire, attached to the frame by a 

 clamp at the lower end, and at the upper end attached to a clamp, 

 which is in turn attached to the surrounding envelope by a pin. The 

 wire is thus put under tension instead of compression. Measurements 

 may be made on wires of any diameter large enough to stand the 

 necessary tension. One must of course be sure that the elongation 

 of the wire due to the tension of the spring is not large enough com- 

 pared with the relative deformations produced by pressure to intro- 

 duce any such complication as might arise from a change in Young's 

 modulus under pressure. A simple calculation will enable one to 

 stay on the safe side of this requirement. 



The magnifying power of the lever had to be determined by a special 

 arrangement, it not being possible to measure the dimensions directly 

 accurately enough. An arrangement was made by which a microm- 

 eter screw could advance the short arm of the lever by a known small 

 amount, the micrometer screw being provided with a vernier scale 

 permitting readings to 1/20°. The accuracy obtainable with the 

 special arrangement was considerably better than 1/10%. 





