PEIRCE. — BEHAVIOR OF THE CORE OF AN ELECTROMAGNET. 115 



springs so weak that if the coil circuit be suddenly closed on itself 

 while the coil is in motion, the damping effects of the induced currents 

 will bring the coil almost instantly to rest wherever it may happen to 

 be, and, until the circuit is broken, the coil will keep its position fairly 

 well. Several years ago Dr. R. Beattie ^ showed that if the ends of a 

 low resistance exploring coil (A) be electrically connected with an in- 

 strument of this kind, and if the flux of magnetic induction through A 

 be changed during the time interval T by an amount N, the coil will 

 move from its initial position to a new position through an angle pro- 

 portional to JV and, apart from pivot friction, practically independent, 

 within wide limits, of T. 



Figure 10. 

 Typical record for half a hysteresis loop, given by Dr. Thornton. 



The " quantometer " first made by Dr. Beattie had a coil of twenty- 

 four and a half turns wound on a metal frame and suspended on a single 

 needle point between the poles of an electromagnet ; the ends of the coil 

 dipped into mercury cups fixed to the case of the instrument. In the 

 kind of fluxmeter now common, the coil is hung by a silk fibre (or a 

 quartz thread) from a spring, so as to avoid pivot friction ; a permanent 

 magnet is used, and the current is led into and out of the coil through 

 helices of very fine silver or copper gimp ; the resistance of this gimp 

 is sometimes much greater than that of the coil itself, and for laboratory 

 use it is often well to employ mercury cups, as Dr. Beattie did, so ar- 

 ranged as to minimize the disturbing effects of capillarity. The original 

 quantometer had a resistance of only one ohm. 



Many persons who have attempted to use very strong electromagnetic 

 fields in d'Arsonval galvanometers have found that it is very difficult 



9 R. Beattie, Electrician, Dec, 1902. 



