STRAIN ON THE PHYSICAL PROPERTIES OK MATTEH. 



19 



It may be objected to the above experiments that too weak a current was employed 

 to bring out with sufficient distinctness any sign of what was looked for; but, as has 

 been already mentioned, the reason for employing such a current was to avoid as far 

 as possible heating effects. Moreover, it was evident that the nickel and iron wires 

 were in a state of circular magnetisation under the influence of the current, inasmuch 

 as a very sensible twist was produced by the combined action of the circular 

 magnetisation and the longitudinal magnetisation, resulting from the vertical 

 component of the earth's magnetic force. The current passed up the wires, and by 

 the earth's vertical magnetic force they were longitudinally magnetised with the 

 north-seeking magnetic pole downwards, the combined action of the two magnetising 

 stresses resulting in twisting the nickel wire in the direction of a right-handed screw ;* 

 by properly timing the impulses of the key used for closing the circuit a deflection of 

 eight scale-divisions could be easily produced. With the iron wire the twist was much 

 more perceptible than with nickel, and in the opposite direction, as might be expected 

 from the researches of JOULE and GOBE. By properly timing the impulses of the 

 key an amplitude of no less than 140 scale-divisions could be got up. 



Experiment XVII. 



[October, 1887. It has been remarked that a sustained electric current passing 

 through a wire does not, except by heating, appreciably alter the internal friction. 

 The currents used in bringing out this result, though strong enough to produce very 

 sensible circular magnetisation, were comparatively feeble. As soon, however, as it 

 appeared, from Experiment VII., that the magnetising stress need only be applied 

 for an instant in order to produce an effect, it was resolved to ascertain to what 

 extent the internal friction could be influenced by circular magnetisation produced by 

 sending an intermittent current through the wire, the current being reversed in 

 synchronism with the torsional vibrations of the wire at the end of each swing of 

 the latter, and instantly removed again after each reversal. The vibration-period 

 of the wire was 4725 seconds. 



Current in C.O.8. nniU lined to 

 circularly magnetite. 







0-0199 

 0-0345 

 0-0616 

 0-1060 

 0-2056 



Logarithmic decrement due to 

 internal friction. 



000845 

 001102 

 001210 

 001564 

 002624 

 004591 



* This shows a shortening in the direction of magnetisation, and therefore the result is in accordance 

 with that arrived at by Professor BARRETT, who has proved that nickel contracts in the direction of 

 magnetisation. 



D 2 



