Sections 29 and 30. Modifications of the Apparatus. 67 



three wires leading to the bomb, IV, fig. 14, does not need to be insulated 

 and is brought out through a small hole drilled in the wall of the bath 

 near the top. The other two, W x and W 2 , are drawn separately into asbes- 

 tos tubing and then brought out through one of the condenser tubes. 



There is such a large mass of metal in the bath that heating it by a 

 burner below would be very slow ; for this reason, an electrical heating coil 

 of nickel-steel wire, 1.6 mm. in diameter, is, as shown, wound on the out- 

 side of the bath, the latter being first covered with a thin layer of mica and 

 a single thickness of asbestos cloth. Two hundred and twenty volts, 

 direct current, placed directly on the terminals of this coil, gives a cur- 

 rent of about 18 amperes. To avoid possible superheating, the electric 

 current is used only to raise the temperature of the bath, the temperature 

 being subsequently maintained by the use of a single gas flame beneath. 



The giving up of the glass cover made it no longer possible to see 

 directly the height of the vapor level in the bath ; but this was readily deter- 

 mined by blowing a little water from a wash bottle onto one of the con- 

 denser tubes, for there is a hissing sound only when the water strikes at 

 or below the vapor level. 



The heater is cooled by passing first compressed air and then water 

 through a spiral of seamless copper tubing (6 mm. in bore), located a few 

 centimeters above the bottom, and by removing the asbestos board on the 

 top of the bath and directing a fan on the cover. 



30. A LIQUID BATH FOR THE ROTATING BOMB. 



This bath has been used principally for measurements at 18, but is also 

 adapted to those at temperatures up to 100. It consists of a copper can, 

 25 cm. in diameter and 33 cm. high, with brass pieces bolted inside to the 

 walls for supporting the bomb, just as in the vapor bath described in the 

 preceding section. Rapid heating is provided for electrically by a heating 

 coil wound on the outside, and a constant temperature is maintained by 

 hand regulation of the current in a small resistance coil placed inside, just 

 off the bottom and in direct contact with the liquid. The bath is cooled 

 by water blown through a helix, consisting of 10 convolutions of seamless 

 copper tubing 4 cm. in bore, which fits friction-tight inside the bath. 

 To hold the temperature down to 18 in summer it is so arranged that the 

 water can first be run through a few turns of lead pipe immersed in an 

 ice bath and then delivered to the coil in the bath drop by drop through a 

 sight feed ; while for rapid cooling the water can be delivered directly 

 from the mains to the coil in the bath. The copper can is well jacketed on 

 the outside with an asbestos composition and is filled with commercial 

 xylene, which is vigorously stirred by a small propeller run by an electric 

 motor. 



