PEIRCE. — BEHAVIOR OF THE CORE OF AN ELECTROMAGNET. 141 



testing room, but it is fairly easy to attach extra weights to the 

 suspended system (Figure 35) of a good d'Arsonval or Thomson Mirror 

 galvanometer which shall so increase the moment of inertia that the 

 time of swing shall be lengthened to five or ten or twenty minutes. 

 With an instrument thus modified it is usually possible, by changing 

 the intensity of the current in the exciting coil by small steps, to deal 

 satisfactorily with very large masses of iron. It is of course desirable 

 to use a rather high electromotive force in the exciting coil in order 



TABLE III. 



to make the building-up time short, and to reduce the current to the 

 desired strength by introducing extra non-inductively wound resistance 

 into the external circuit. In order to test this matter thoroughly, I 

 measured with great care, by aid of a modified Rubens-du JBois 

 "Panzer Galvanometer," the flux changes in the core of the magnet Q 

 (the area of the cross-section of which is more than 150 square centi- 

 meters), corresponding to a hysteresis cycle for an excitation of 1812 

 ampere turns. I then determined the same total flux change by 

 means of planimeter measurements of the areas under a long series of 



