64 



On an Induction- Currents Balance. 



[May 15, 



IX. There is a marked difference of the rapidity of action between 

 all metals, silver having an intense rapidity of action. The induced 

 currents from hard steel, or from iron strongly magnetised, are much 

 more rapid than those from pure soft iron ; the tones are at once 

 recognised, the iron giving out a dull, heavy smothered tone, whilst 

 hard steel has tones exceedingly sharp. If we desire to balance iron, 

 we can only balance it by a solid mass equal to the iron to be balanced. 

 ~Ro amount of fine wires of iron can balance this mass, as the time of 

 discharge of these wires is much quicker than that of a larger mass of 

 iron. Hard steel, however, can be easily balanced not only by steel 

 but by fine iron wires> and the degree of the fineness of these wires 

 required to produce a balance gives a very fair estimate of the pro- 

 portionate time of discharge. The rapidity of discharge has no direct 

 relation with the electrical conductivity of the metal, for copper is much 

 slower than zinc, and they are both superior to iron. 



X. The instrument shows a marked difference in all. metals, if sub- 

 jected to different temperatures. The value is reduced in non-mag- 

 netic metals, and this we should expect from the known influence of 

 temperature on the electrical conductivity ; but in the case of iron, 

 steel, and nickel (as it has already been remarked by many), the con- 

 trary takes place, namely, a far higher degree of magnetic conductivity. 

 A bar of soft iron, whose value at the temperature of the room, 20° C, 

 was 160, became on heating it to 200° C. 300, that is to say, its value 

 was nearly doubled. A bar of pure nickel, whose value at 20° was 

 150, became on heating it to 200°, 320 ; thus, in the case of nickel, 

 its value for magnetic conductivity was more than doubled, and at 

 this heat it surpassed the chemically pure iron at the same heat, 

 giving a magnetic value of 320 against 300 for the iron, but at the 

 normal temperature of 20° the iron had more magnetic power of con- 

 duction than nickel. Heating nickel, by simply plunging it into boil- 

 ing water, increased its force from 150 to 250; plunging this same bar 

 into ordinary cold water reduced its value to 130 ; thus the mere 

 difference of the normal temperature of the air in the room and water 

 which had been in this room some hours produced 20° of difference. 

 In fact, I found that the radiant heat from the hand would raise the 

 magnetic value several degrees, and thus nickel may be regarded as a 

 magnetic thermometer far more sensitive than the ordinary mercurial 

 Centigrade. 



The instrument also measures the electrical resistance of wires or 

 fluids. In order to make it do this, we have only to place the re- 

 sistance to be measured across the two wires of one induction coil and 

 on the other known resistance units. In this way we can produce a 

 perfect balance, for it then becomes an induction bridge, the results 

 and modes of testing of which are somewhat similar to Wheatstone's 

 bridge. 



