310 Mr. E. H. Hull on the new Action of 



being known, its sensitiveness (that is, the significance of its 

 readings in absolute measure) was determined whenever the 

 strength of the magnetic field was to be found. This was 

 effected by means of an earth inductor placed in circuit with 

 the galvanometer, and the test-coil used with the magnet. 

 Thewdetermination of the strength of the magnetic field there- 

 fore involves two series of observations, one with the earth 

 inductor and one with the test-coil. 



3rd. A determination of the sensitiveness of the Thomson 

 galvanometer. — This was done by sending through it a current 

 of known strength obtained by shunting the current from a 

 Bunsen cell, the main current being measured with a tangent- 

 galvanometer. 



4th. The main experiment. — The primary current through 

 the metal strip measured with the tangent galvanometer just 

 spoken of, and the effect of reversing the magnet observed on 

 the scale of the Thomson galvanometer. 



5th. Another determination of the sensitiveness of the Thom- 

 son galvanometer. — Method as described above. 



6th. Another series of observations with the test-coil. 



7th. Another series of observations with the earth inductor. 



8th. Another determination of the direct action of the mag- 

 net on the Thomson galvanometer. 



If, as was usually the case, several series were to be made 

 with the same plate in one day for the purpose of using pri- 

 mary currents of various strengths, the sensitiveness of the 

 Thomson galvanometer was tested before each main series of 

 observations and after the last. 



The mean of two values found for the sensitiveness of the 

 Thomson galvanometer was, of course, taken to be the sensi- 

 tiveness during the series of observations intervening. It was 

 not found necessary to determine the strength of the magnetic 

 field more than twice during a half-day's observations. 



In working up these observations the following formula 



applies :— 



. <1> 



sin or 7 i 



7460H— % ^^ 

 . <P' wt 



MxV Sm Y 



E x dktxm®pr 



Wm 



M, V, and W have been already defined. 



7460 = twice the integral area of the earth inductor divided 

 by the integral area of the test-coil. Twice the simple 



