122 Mr. E. L. Wills on the Effect of 



The ballistic galvanometer is standardized by means of 

 the solenoid D, the two-way key K t being so arranged that 

 the current passes through the primary o£ the solenoid instead 

 of the coil PC ; the secondary coil, wound on a tube inside the 

 solenoid, is connected in series with the secondary coil SC of the 

 specimen, and is kept continuously in the secondary circuit. 



The electrodynamometer is standardized by suddenly turning- 

 over an earth-inductor EI placed in the secondary circuit, 

 while a known current is passing through the primary 

 circuit, and observing the " throw " produced in the movable 

 coil B. 



When measuring hysteresis the earth-inductor is replaced 

 by a coil of the same resistance as the earth-coil but wound 

 non-inductively . 



The movable coil is quickly brought to rest by sending a 

 very small current through it, in the right direction, while 

 the primary current is flowing through the fixed coils. This 

 is done by connecting one of its terminals to any point in a 

 closed circuit consisting of a Leclanche cell, a fairly high 

 resistance, and a piece of german-silver wire, and the other 

 terminal through a tapping-key to a suitable point in the 

 german-silver wire. The resistance R 3 is permanently con- 

 nected in series in the secondary circuit in order to reduce 

 the sensitiveness of the ballistic galvanometer or electro- 

 dynamometer when necessary. 



If the maximum current be kept constant, but the resistance 

 of each of the parts into which the primary circuit is divided 

 by the key F increased in any ratio by increasing R x and R 2 

 respectively, then dC/dt will be increased in the same ratio. 

 The main part of the variation in the primary resistance was 

 made by varying R 1? the small resistance R 2 being used to 

 adjust the current to any definite value. 



The secondary resistance S was varied from about 23 ohms 

 to 523 ohms, and the throws of the dynamometer coil for each 

 induction were, as nearly as could be read, inversely propor- 

 tional to the secondary resistance ; the term 6R/S in equation 

 (4) was therefore negligible in comparison with W + «R. 

 We thus have 



W=U-aR (5) 



If, for each value of the maximum magnetizing force, we 

 measure U 1? U 2 , U 3 , the values of U when the voltages of 

 the battery in the primary circuit are E, 2E ; and 3E 

 respectively, and the resistances of the primary circuit are 

 R, 2R, and 3R respectively, then 



W = 2U 1 -U 2 = 3U 2 -2U 3 . 



