494 



Scientific Proceedings, Royal Dublin Society. 



The curves in fig. 5 show liow the maximum twist — when the wire is 

 in a longitudinal magnetic field of 2'5 units — varies with the hardness of the 

 wire, and when the longitudinal load on the wire is varied. The top curve 

 represents the results obtained with a load of 0'5 x 10* grammes per sq. cm., 

 the middle curve with a load of 1'5 x 10* grammes per sq. cm., and the lowest 

 curve wlien a load of 3"0 x 10' grammes per sq. cm. was on the wire. 



770 790 aiO S30 



Fig. 5. — Rigidity (grammes per sq. cm. x 10^). 



These curves show that as the hardness is increased from Ho to H2, the 

 twist is decreased 16 per cent, for the small load, about 29 per cent, for the 

 high load, and about 22 per cent, for the intermediate load. The same 

 general forms of curves are obtained if the results are plotted for any other 

 longitudinal magnetic field. In fact, as the load on the wire and longitudinal 

 magnetic field round it are increased, the relation between the rigidity and 

 twist tends toward a straight line, and in the lower curve of fig. 5 

 the twist is decreased about 53 per cent., when the hardness is increased 

 from Hq to H3 on tlie arbitrary scale. 



Experiments were now made to find out how the twist of the free end of 

 the wire varied when the diameter or cross-sectional area of the wire was 

 changed. The wires, as received from the manufacturer,' were all in the 

 same physical state, and had all received the same heat-treatment ; that is, 

 they were put in an annealing furnace and raised to a bright, cherry-red heat, 

 then allowed to cool naturally until they could be handled. 



' For this set of experiments many attempts were made, and various methods tried to get the 

 different-sized wires all in the same soft state Ho, but with no success owing to the great length of 

 wire employed. It was found, however, on inquiry, that the wires as delivered by the manufacturer 

 were all in the same physical state as regards heat -treatment, and they were therefore tested in that 

 state. 



