508 Prof. Riicker. On the Magnetic Permeability of 



FIG. 2. 



field. Iii this way an oscillation-period of 25 seconds could be ob- 

 tained, although it was found in practice to be unnecessary to increase 

 it beyond about 10 seconds. At Jc there is a key which divides the 

 -current, part of which goes through, the two large solenoids SS, flow- 

 ing in the same direction in each, and thence through the galvano- 

 meter G back to the battery. Another part goes through the 

 reversing key K to the small deflecting solenoid s and thence through 

 the Post Office bridge P back to the battery. The resistance used in 

 the Post Office bridge was about 200 ohms, while that of the main 

 circuit was about 10 ohms, so that the amount of current shunted 

 was comparatively small ; nevertheless, to prevent any error being 

 thus caused, another shunt circuit, of resistance R, equal to that in 

 the Post Office bridge, was introduced, through which the current 

 passed when the small solenoid was not in use. 



As the effects produced by the two principal solenoids on the mag- 

 netometer were no t exactly balanced when they were placed at equal 



