1881.] Molecular Electro- Magnetic Induction. 533 



critical point of 1 millim. hard iron wire was 20° of torsion, but in 

 hard steel it was 45°. 



Longitudinal strains do not produce any current whatever, but a 

 very slight twist to a wire, under a longitudinal strain, produces its 

 maximum effects : thus, 20° of torsion being the critical point of iron 

 wire, the same wire, under longitudinal strain, required but from 10° 

 to 15°. It is very difficult, however, to produce a perfect longitudinal 

 strain alone. I have, therefore, only been able to try the effect of 

 longitudinal strain on fine wires, not larger than 1 millim. in diameter, 

 but as in all cases, no effect whatever was produced by longitudinal 

 strain alone, I believe none will be found if the wire be absolutely free 

 from torsion. The molecules in a longitudinal strain are equally under 

 an elastic strain as in torsion, but the path of their motion is now 

 parallel with its wire, or the zero of electric inductive effect, but the 

 longitudinal and transverse strains of which the compound strain 

 is composed, react upon each other, producing the increased effect due 

 to the compound strain. 



The sonometer is not only useful for showing the direction of the 

 current and measuring it by the zero method, but it also shows at 

 once if the current measured is secondary or tertiary. If the current 

 is secondary its period of action coincides with that of the sonometer, 

 and a perfect balance, or zero of sound, is at once obtained, and its 

 value in sonometric degrees given, but if the current is tertiary, no 

 zero is possible, and if the value of the tertiary is 60°, we find 60° 

 the nearest approach to zero possible. But by the aid of separate 

 induction coils to convert the secondary into a tertiary, a perfect zero 

 can be obtained if the time of action and its force correspond to that 

 which we wish to measure. 



If I place a copper wire in the balance and turn the coils at an 

 angle of 45°, I obtain a current for which the sonometer gives a perfect 

 zero at 50°, proving, as already said, that it is secondary. If I now 

 replace the copper by an iron wire, the coil remaining at 45°, I have 

 again exactly the same value for the iron as copper, viz., 50°, and in 

 both cases secondary. Now, it is evident that in the case of the iron 

 wire there was produced at each passage of the current a strong 

 electro-magnet, but this longitudinal magnetism did not either change 

 the character of the current or its value in force. 



A most beautiful demonstration of the fact that longitudinal mag- 

 netism produces no current, but that molecular magnetism can act 

 equally as well, no matter the direction of the longitudinal magnetism, 

 consists in forming an iron wire in a loop, or taking two parallel but 

 separate wires, joined electrically at their fixed ends, the free ends 

 being each connected with the circuit, so that the current generated 

 must pass up one wire and down the adjacent one. On testing this 

 loop, and if there are no internal strains, complete silence or absence 



VOL. xxxi. 2 p 



