233 



in diameter and has near tlie lK)ttoni a liollow wooden cjiinder (II), upon 

 which rests a small marble, which acts as a valve. The piston of the 

 pump is made of a smaller tube, which is just large enough to slip easily 

 inside the J-inch tube. The valve in it is of the same type as the one at 

 the bottom of the tube. The piston ro<Is extend through the central hole 

 at the top of the heating pipe, and are attached to a lever which is piv- 

 oted to a supi>ort fastened to the oaken partition. 



Above the pipes is mounted a switchboard (Fig. 1), containing the 

 necessary measuring instruments. The ammeter on the right side of the 

 switchboard measures the current used in heating the covered pipe, and 

 the one in the upper central part of the board measures the current used 

 in heating the uncovered pipe. The two coils are in multiple circuit, and 

 when switch C is closed current passes through both coils, the amount 

 in each coil being regulated by the two rheostats. The upper rheostat 

 controls the current in the covered pipe, and the lower one controls the 

 current in the uncovered pipe. When the switch on the left side of the 

 board is thrown, closing circuit A, the voltmeter is connected to the 

 terminals of the coil in the unjacketed pipe, and when thrown, closing 

 circuit B, it is connected to the terminals of the jacketed pipe. Switch 

 B is in multiple circuit with an impedance coil, and may be used when 

 a large circuit is needed in the heating coils. 



Each of the heating coils has a resistance of about (5.5 ohms, the im- 

 pedance coil a resistance of about 9.3 ohms, and each rheostat has a re- 

 sistance of 7.5 ohms when all is used. At the outset of a measurement 

 the resistance of the rheostats is thrown in, switch D is closed, thou the 

 sides of the rheostats moved until the current in the covered pipe is S 

 amperes. The oil in the pipes is stirred by means of the pumps. When 

 the temperature of about 100° C. is reached switch D is opened, and 

 while the oil is vigorously stirred the current is regulated until the tem- 

 perature of both pipes is kept at the same constant value. After the two 

 pipes have kept at the same constant temperature for about ten minutes, 

 the temperature of each oil bath, the voltage at the terminals of each coil, 

 and the current in each coil, is read. 



A record of the test is as follows : 



Outside diameter of pipes, 3.5 in. 

 Length exposed, 3G.0 in. 

 Temperature of surroundings, 23.1° C. 



