1879.] 



On an Induction- Currents Balance. 



61 



when its wire is parallel to those on coils bnt if it is held at right 

 angles to these wires, no sound whatever is heard, and the balance 

 remains perfect. The same thing occurs with disks of all non- 

 magnetic metals, and a disk of metal placed perpendicular to the coils 

 exerts no influence whatever. The contrary result takes place with a 

 spiral of iron wire or disk of iron; the induced current circulating in 

 the spiral is at its maximum when the spiral lies flat or parallel with 

 the coils, being reduced to nothing when at right angles, but the dis- 

 turbance of the induction balance is more than four times as great 

 when the spiral is perpendicular to the wires of the coils as when 

 parallel with the same. That this result is simply due to the property 

 of magnetic bodies of conduction of magnetism, we shall see in some 

 following experiments. 



That the currents in non-magnetic metals travel in a circle corre- 

 sponding to that of the primary coil, may be seen with spongy gold. 

 In its first extremely divided state it falls below our zero of hearing, 

 on slightly shaking the bottle we have 2 3 as its value, on pressing 

 it its value rapidly increases with the pressure, until when formed into 

 a solid disk its value becomes 117 : . 



V. The instrument proves, that a very remarkable difference exists 

 in bars of iron of the same exact form and size, but of different 

 origin or treated in a different manner : in point of fact, no two 

 bars, cut off of the same rod, and treated alike, are exactly of the same 

 value, or induce a complete balance. 



Mr. Stroh, the eminent instrument maker, has kindly furnished me 

 with numerous samples, varying' in value in degrees of the sonometer 

 from 100 to 160. 



Chemically pure iron was found to be the best, but still very 

 slightly superior to ordinary iron, which had been drawn into a wire 

 of the required thickness. The fibrous condition thus developed is 

 highly favourable (if the iron is softened by heat) for the conduction 

 of magnetism. From numerous examples I select a few indicative 

 values : — 



Softened. Tempered. 



Chemically pure iron 160 130 



Forged soft iron 150 ...... 125 



Wire- drawn iron. , .... .... lo6 120 



Cast steel „ 120 ...... 100 



VI. As yet the instrument has given no indications of molecular 

 change produced by magnetism in non-magnetic bodies, bat the great 

 change which takes place in all magnetic bodies, except hard-tempered 

 cast steel, indicates that a molecular change of structure, analogous to 

 that of tempering, takes place upon iron, steel, and nickel. 



If we place a disk of iron in one of the coils, we find that the 

 balance is destroyed, and that the iron has weakened the induction by 



