488 Scientific Proceedings^ Royal Dublin Society. 



wire ; and when the length of the wire is halved, the mean maximum twist 

 is decreased about 50 per cent, in each case when the three loads are used. 



This straight-line relation between tlie length of the wire and the twist 

 also holds very approximately for fields higher than 2'5 units, showing that 

 the curves are practically parallel. 



In the same way by plotting the values of the lengths of the wires as 

 abscissae against the corresponding values of the areas of tlie cyclic curves as 

 ordinates, when the wires were subjected to the three different longitudinal 

 loads, the points will be found to lie in a straight line iu the tliree cases, and 

 these lines also when produced will pass through the origin and sliow that 

 when the length of the wire is halved the circular magnetism when the wire is 

 in a magnetic field of 2"5 units is decreased about 48 per cent. 



Again, if for the three different lengths of wire we plot the values of 

 the load as abscissae, and as ordinates the corresponding values of the circular 

 magnetism or areas of the cyclic curves in a magnetic field of 2'5 units, the 

 points will be found to lie in a straight line in each of the three cases, and 

 show that when the load is increased sis times the circular magnetism is 

 decreased about 35 per cent. Also for the three different lengths of wire 

 by plotting the values of tlie load on the wire as abscissae, and as ordinates 

 the corresponding values of the maximum twist when the wire is in a 

 magnetic field of 2'5 units, the points will be found to lie in a straight line in 

 each of the three eases, and show that when the load on the wire ia increased 

 six times the mean maximum twist is decreased about 27 per cent. 



These three latter straight lines, when produced, cut the axis of abscissae 

 at a mean point representing a load of 8'2 x 10° grammes per sq. cm., which is 

 very fipproximately the elastic limit of the wire, and shows that with tlie 

 given current density in the wire and in the given magnetic field of 2'5 units 

 if the wire be stretched near to its elastic limit, there will be little or no 

 twist on the free end. This confirms an observation made in previous 

 work,' where it was shown that no transient current would be produced 

 by twisting a wire placed in a longitudinal magnetic field if the load on 

 the end of the wire was such as to stretch it to, or nearly to, the elastic limit. 



With the arrangement of apparatus used in these experiments it was 

 not possible to put such a large weight on the end of the wire ; it was 

 reckoned, however, that a steel wire of the same diameter, with a very 

 small weight on the end to keep it straight, would be at least as hard 

 as the iron wire when loaded near to its elastic limit. A steel wire 

 of rigidity 830 x 10* grammes per sq. cm. and electrical conductivity 9'6, 



> Scient. Proc. Eoy. Dub. Soc, 1909, vol. xii, p. 110. 



