NOYES AND COOLIDGE. — ELECTRICAL CONDUCTIVITY. 171 



of the heater. Repeated trials showed that the temperature of the bath 

 throughout the space surrounding the bomb varied less thau 0.1°, so that 

 the exact position of the thermometer made no difference. Care had to 

 be taken, however, that the mica shield above the bomb did not come iu 

 contact with the thermometer stem, thus allowing the condensed vapor 

 coining from the shield to run down and cool the bulb. It was feared 

 that the vapor condensing on the upper part of the thermometer itself 

 would have the same effect ; but this was proved not to be the case by 

 fastening a small inverted watch glass about midway on the thermometer 

 stem ; this carried off the drip from the upper part of the stem, but did 

 not affect the reading. 



■»■ 



III. Details of the Construction of the Bomb. 



The shell is made of the softest crucible-steel obtainable, because duc- 

 tility is desired rather than high tensile strength.* Extra weight is not 

 objectionable here, as it would be in the case of a calorimetric bomb ; 

 moreover, fear was entertained that a high-carbon steel might be weak- 

 ened by the repeated heating and cooling to which the bomb was to be 

 subjected. The shell was designed for approximately equal strength 

 throughout. The existing data on the tensile-strength of steel at the 

 high temperatures involved only suffice to show that it is considerably 

 weaker than at ordinary temperature. Owing to this fact, and to the fur- 

 ther fact that it was uncertain to how high a temperature the observa- 

 tions would ultimately be carried, a factor of safety of 20-fold was 

 allowed, based upon the tensile strength cold and the critical pressure of 

 water- vapor. The large nut, C, has an ordinary V-thread of 18 turns to 

 the inch. To tighten the nut, the lower part of the bomb is held at R, 

 which is hexagonal, by a wrench bolted to a firm table ; while a second 

 wrench, with an effective length of 46 centimeters, is placed on the hex- 

 agonal part, R', of the nut itself. In this way sufficient pressure can be ex- 

 erted on the gold packing ring to make the metal of which it is composed 

 actually flow into the groove beneath, filling any little scratches or other 

 depressions which may exist in the latter. Since there is a certain thick- 

 ness of gold and platinum interposed between the cover and the lower 

 part of the bomb, and since these both expand less than steel upon heat- 

 ing, it becomes necessary to use a compensating brass washer, W, between 

 the nut and the cover. The proper thickness can be calculated from the 



* Bessemer steel would probably be somewhat safer and will be used in future 

 work. 



