ELECTRICAL RESISTANCE UNDER PRESSURE. 133 



At 0° the pressure coefficient was - 0.06134, and at 90° -O.OelS?. 

 The temperature coefficient between 0° and 90° was 0.000163. 



"Chromel B." This alloy is much similar to "Chromel A." It 

 is made by the same concern, and has a composition of 85% nickel and 

 15% chromium. Like the p^e^'ious material it was furnished in the 

 form of wire 0.005 inches in diameter, and was double covered with 

 silk insulation by the Xew England Electrical Works. It was also 

 wound into a coreless toroid of approximately 100 ohms resistance at 

 0°. It was seasoned for temperature at the same time as the " Chromel 

 A" by four hours at 135°, but was in addition seasoned for pressure 

 by a single application of 12000 kg. at room temperature, and a single 

 application of 2000 kg. after mounting ready for the measurements. 



The general character of the results is the same as for " Chromel A." 

 The coefficient is not quite so small, and the results were considerably 

 more regular. This was in part due to the choice of a less viscous 

 transmitting medium. The relation between pressure and resistance 

 is linear within the limits of error. Two series of measurements were 

 made, at 0° and 95°, to a maximum pressure of 12000 kg. At 0° the 

 maximum departure of any single reading from the linear relation was 

 1.5% of the maximum effect, and at 95° it was 1.9%, except for the 

 zero, which showed a displacement of 3.5%. 



At 0° the average pressure coefficient was — 0.06158, and at 95° 

 — 0.06169. The average temperature coefficient of resistance at 

 atmospheric pressure between 0° and 95° was 0.000212. 



"Chromel C." This alloy is also intended for high resistance 

 heating units. It is made also by the Hoskins Co., but unlike Chromel 

 A and B contains some iron in addition to nickel and chromium. The 

 exact composition is Fe 25%, Ni 64%, and Cr 11%. The wire was 

 0.005 inches in diameter, double silk covered, and wound into a core- 

 less toroid of such dimensions as to have at 0° a resistance of 178 ohms. 

 It was seasoned by one preliminary application of 12000 kg., and after 

 connecting to the pressure apparatus by three applications of 2000 kg. 

 Measurements were made on the Carey Foster bridge, as usual with 

 materials of high resistance. 



Three runs were made, at 0°, 52.24°, and 95.88°. The variation 

 of resistance is not throughout linear with pressure and temperature, 

 but shows departures in abnormal directions. The departures from 

 linearity do not run uniformly, so that it was not possible from the 

 three series of readings to construct a table of resistance which could 

 be used by interpolation to obtain the resistance to the limit of accu- 

 racy at any temperature and pressure within the range. This alloy 

 would merit further study for its own sake, but an elaborate investi- 



