234 PROCEEDINGS OF THE AMERICAN ACADEMY. 



The shape of the iron is an important consideration. That all un- 

 necessary difficulties might be avoided, such as would be introduced by 

 the poles of a broken magnetic circuit, the toroid was chosen, for my 

 work, as the simplest and consequently the most suitable form. 



The arrangement of the apparatus is briefly as follows. The primary 

 of the toroid, TP, Figure 3, to be investigated, is connected in series 

 with an amperemeter, A, a set of resistance gaps, K.G, into which any 

 desired resistance coil could be introduced, a rheostat, R, a reversing 

 switch, 0, and a set of ten storage cells. The double switch, N, 

 allowed me to change from the primary of the toroid to the primary 

 of the calibrating solenoid, CP. The switch, M, was for the purpose of 

 introducing a resistance, K, approximately equal to that of the primary 

 of the toroid, when it was desirable to reduce the field in it to zero 

 without breaking for any length of time the flow of current from the 

 battery. This device kept the current more nearly constant than 

 would have been otherwise possible. The secondary circuit consisted 

 of the ballistic galvanometer, BG, in series with the secondary of the 

 toroid, TS, the secondary of the calibrating solenoid, CS, and the 

 switch, L. This last permitted me to throw the galvanometer into 

 the damping circuit. The flux changes produced in the iron core were 

 measured by the throw of the ballistic galvanometer. 



Description of the Apparatus. 



The usual form of ballistic galvanometer with a period of aljout 15 

 seconds is not suited for this work. The total flux change in the 

 toroid is not completed before the coil has swung considerably out of 

 its zero position ; and consequently the angular rotation of the coil 

 is no longer proportional to that flux change. For my purposes I con- 

 structed an instrument similar to one used by Professor Peirce.** 



The coil of the galvanometer, which was of the Ayrton-Mather type, 

 was loaded by two brass balls to increase its moment of inertia. The 

 whole was suspended by steel gimp ; the current was conducted to 

 and from the coil, of which the ends were soldered to the two vertical 

 wires, by spirals of copper gimp similarly attached. 



The increase in the inertia of the suspended system due to the two 

 brass balls increased the period of the galvanometer in this instance 

 to 65 seconds. As it was found by experiment that, if the secondary 

 circuit were closed one half a second after the primary circuit, there 

 was no perceptible deflection of the galvanometer, the period was re- 



" These Proceedings, 44, 283 (1909). 



