160 Dr. E. Taylor Jones on 



agreed within J and J per cent., respectively, with the values 

 obtained electromagnetically. 



A series of observations was then made of the tractive force 

 and at the same time of the induction-throws caused by the 

 coils falling from the isthmus, the electromagnet being excited 

 by various currents ranging from to 25 amp. 



One great difficulty in measuring the tractive force was due 

 to the fact that the iron bar, and, more especially, the steel 

 bar (which in the process of hardening might easily be slightly 

 bent) , often got jammed in its socket. If the bar was pulled 

 exactly in the direction of its axis and that of its socket the 

 jamming was avoided, and the friction was very slight. This 

 was determined by removing the other pole-piece and demag- 

 netizing the electromagnet as far as possible in order to 

 remove all magnetic action. By properly guiding the brass 

 rod M, the position was then found in which the weight 

 necessary to pull the bar out was a minimum, and this was 

 found to be less than 5 grammes weight, which is negligible 

 in comparison with the tractive forces obtained later. 



But if the bar was pulled only slightly to one side, it 

 became jammed in its socket, and about 700 grins, weight 

 more were necessary to pull it out. In all subsequent 

 measurements therefore the position was found, by properly 

 guiding the brass rod, in which the tractive force was a 

 minimum, and this was taken as the magnetic tractive force. 

 The question as to whether the magnetization had any appre- 

 ciable effect on the friction was tested by pushing the longer 

 bar C 2 through both pole-pieces (C^ being removed), so that it 

 lay in a symmetrical position of minimum potential energy. 

 No effect on the friction could be detected when the exciting 

 current was made or broken*. 



For the graphical evaluation of the integral I Hdl it 



was 



necessary to have the magnetization-curves for the iron and 

 steel. 



The higher values of H and I were already known from the 

 isthmus experiments. For the determination of the earlier 

 parts of the curves, two bars were used which were cut from 

 the same pieces as the isthmus bars. The curves were then 



* This arrangement, in which no gap existed in the isthmus, afforded 

 at the same time a means of testing how far the gap in the usual 

 arrangement caused disturbance by spreading the lines of induction. It 

 was found that in the strong fields here used this was a considerably 

 smaller source of error than might be supposed after certain experiments 

 at less intense fields. 



