PEIRCE. — MAGNETS OF HARDENED CAST IRON. 



11 



of Crescent Drill Rod, treated with the castings, furnished the dotted 

 curve in the same figure. A careful comparison of the curves of Figures 

 3 and 4, obtained with the cliilled castings — wliich were chosen from 

 the lot wholly at random — will show that they are magnetically almost 

 indistinfTuishable ; the likeness of the Crescent Drill Rod, chosen also at 

 random, to the castings is again shown by the curves of Figure 4. 



We have used cast-iron magnets of many different forms, a few of 

 which only are shown in Figure 5 ; the shapes marked 1, 2, 3, 6 have 



Figure 4. 



been employed, with the long way of the opening between the poles ver- 

 tical, in d'Arsonval galvanometers, while a number of rather thin plates 

 of the shape marked 8 have been used together in other instruments 

 of the same kind. Magnets of the shapes mai-ked 5 and 7 produce the 

 artificial fields in some mirror needle galvanometers used as voltmeters. 



The mean temperature coefficient (K) between 10° C. and 100° C. of 

 hardened magnets of the shape 1, which weigh 12.")0 grams apiece, is about 

 0.r,0036; K is the mean loss per degree of the intensity of the magnetic 

 field at any definite point between the poles — when the magnet is heated 

 from 10° to 100° — expressed in terms of the field intensity at the point 

 at the lower temperature. For magnets of the shapes 3 and G — which 

 weigh 260 grams and 500 grams respectively — K has the values 



