RESISTANCE DEVICES 



145 



coil of 5 units and the succeeding values by employing this coil 

 in series with the four-step decade. Only one plug is required. 



The difference between Smith's and Feussner's arrangements 

 is obvious. 



In Northrup's arrangement, four coils having denominations 

 1, 3, 3, 2, units are used. In Fig. 70, I, II, III, IV, V, are termi- 

 nal posts and taps which may be connected as desired. If all the 

 coils are used in series the resistance is 9 units. The other values 

 are obtained as shown below. 



The construction necessary for carrying out this scheme by the 

 use of a single plug is shown in Fig. 70. 



RHEOSTATS 



Water Rheostats. To control a small current and to be able 

 to give it any value between zero and a maximum, the arrange- 

 ment shown in Fig. 71 may be used. 



The compensating cell renders it possible to bring the current 

 in the derived circuit smoothly down to zero. If the cell is not 

 used, there will be a sudden change in the current when the 

 electrodes a and b are brought into contact. 



Water rheostats are commonly used for absorbing energy 

 during tests of electrical machinery. The "water barrel," 

 shown in Fig. 72, is convenient when small amounts of power are 

 to be dealt with. 



A stout wooden barrel is used. An ordinary cast-iron stove 

 10 



