Peirce — Induction Coefficients of Hard Steel Magnets. 349 



steel in order to find out whether any of these were much 

 superior to magnets of similar shape made of drill rod. The 

 determinations were made^ with the aid of an induction bench 

 two meters long furnished with two pairs of solenoids so 

 placed and connected that a steady current passing successively 

 through the coils of either pair would not deflect the mag- 

 netometer needle. This needle was closely surrounded by a 

 small coil through which, in experimenting upon a magnet, a 

 steady current could be sent just strong enough to balance 

 the deflecting force of the magnet when placed in the center of 

 any of the solenoids. The general arrangement of the appa- 

 ratus appears in figure 1. The deflections of the magnetome- 



ter needle were measured by means of a telescope and scale ; 

 the strength of the weak current sent at any time through the 

 solenoids, by a sensitive voltmeter in multiple arc with a 

 standard resistance in the main circuit. The strength of the 

 earth's magnetic field at the center of the magnetometer was 

 known. 



After an unmagnetized specimen of hard steel had been 

 inserted in the center of one of the solenoids, at a known dis- 

 tance from the magnetometer needle, a steady current so weak 

 as not to leave any perceptible permanent magnetismf in the 

 steel was sent through the solenoid. This current was reversed 

 twenty times and then the deflection of the needle was meas- 

 ured for each direction of the current. With the customary 

 correction;]: for the effective length of the steel, it was easy to 

 compute the moment of the induced magnetism so that a sim- 

 ilar measurement made in one of the other pair of solenoids 

 would give practically the same result. Sometimes a number 

 of measurements were made with the same specimen, using 

 different magnetizing currents in order to make sure that the 



* Maxwell: Treatise on Electricity and Magnetism, § 457. 



WiDkelmann : Handbuch der Physik, III, ii, p. 69. 



Joule: Proc. Phil. Soc , Manchester, 1867. 

 fin the cases of some short pieces, a field of 3<=s* units was not too strong. 

 j:E,iecke: Wied. Ann., 1879. 



