108 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



electric magnet over that of the coil; while by the second 

 theory, when the coil is very close around the iron, the whole 

 action is due to the magnetism of the iron. The natural 

 unit of magnetism to be used in the first theory is that quan- 

 tity which will repel an equal quantity at a unit's distance 

 with a unit of force. On the second theory, it is the number 

 of lines of force which pass through a unit of surface when 

 that surface is placed in a unit field perpendicular to the 

 lines of force. As the result of his novel and very important 

 researches on the effect of heat on magnetism, Rowland states 

 that if it were possible for the magnetism of substance to 

 attain a maximum value, the co-efficient of magnetism by in- 

 duction would become, first, zero, and then negative, and the 

 substance would then become diamasmetic for very hiofh ma^- 

 netizing forces. This principle, announced independently by 

 Maxwell and Rowland, lacks as yet the confirmation of ob- 

 servation, although not contrary to our experience. Our 

 principal hope of confirming it by observation consists in 

 heating some body, and then subjecting it to a very high 

 magnetizing force, for Rowland has shown in the case of iron 

 and nickel the maximum of magnetization of nickel and of 

 iron decreases as the temperature rises, at least between the 

 limits of zero and 220 Centigrade. He finds from observation 

 that if nickel is heated from 15 to 220 Centigrade, the mag- 

 netization will increase if the magnetizing force is small, but 

 will decrease if it is large. In general, as the magnetizing 

 force is increased, the resistance of iron, nickel, and cobalt to 

 magnetization decreases, until a minimum is reached when 

 the metals have attained a magnetization equal to from 24 to 

 30 per cent, of their maximum of magnetization, and after 

 that resistance increased indefinitely. 7 A, XLVIIL, 32. 



IMPROVEMENTS IN THE GRAMME MAGNETO-ELECTRIC MACHINE. 



The magneto-electric machine invented by Gramme, which 

 has within the past two years become quite famous, has re- 

 ceived an important improvement in that ordinary magnets 

 have been replaced by the plate magnets invented by Jam in, 

 which give it a great advantage, not only because of the 

 greater force for the same weight, but because of the ex- 

 treme facility of their construction. These plate magnets 

 can be built up and taken apart in a few minutes, an ex- 



