116 



Scientific Proceedings^ Royal Dublin Society, 



following experiments we may take it that about 226 cms. of the wire was in a 

 magnetic field of uniform strength. 



The circuit was now arranged with the No. 16 iron wire hanging 

 vertically in the centre of the solenoid, and a longitudinal stress, or load, put 

 on the end of the wire of 10= grammes per sq. cm. A current was sent round 

 the solenoid in such a direction as to give, along with the Eartli's vertical 

 force, a total magnetic field of 7 c. g. s. units. 



The wire was then put through a complete oycle, by sending currents 

 through it, increasing by steps up to 2-06 amperes as maximum, and the deflec- 

 tions on the scale or twist read at each step as before. The load on the wire was 

 then increased, and a cycle again taken, and so on for seven different values of 

 the load in all. The cyclic curves were all drawn to the same scale, viz., 2 cms. 

 representing one ampere on the axis of abscissae, and 1 cm. representing ten 

 scale divisions, or twist, on the axis of ordinates. The area in sq. ems. of 

 the complete curve was measured in each case, and also the total twist for 

 the highest current of 2'06 amperes observed. 



The results for a total magnetic field of 7 c. g. s. units are here given in 

 Table YI. 



Tablk VI. 



If we plot the numbers in the first column of Table YI. as abscissae, and 

 the numbers in the second and third columns as ordinates, we obtain two 

 curves which fall to a minimum towards the axis of abscissae, and tlien rise 

 slightly. It will, therefore, be found that when the load on the wire is 

 increased about 70 per cent, the maximum twist is decreased about 16 per 

 cent. ; and for a load of 2'1 x 10^ grammes per sq. cm., or about the -^ih part 

 pf the tenacity of iron-wire, the twist reaches a minimum ; and when the load 



