434 Mr. J. Hopps on the Alterations in the 



" On the Adjustment of Resistance-Coils/' by Prof. Silvanus 

 P. Thompson ; and although some of the difficulties in the way 

 of adjusting resistance-coils were spoken of, yet the difficulty 

 that had given us so much trouble was not once alluded to, 

 either in the paper or in the discussion which followed. At 

 the close of the proceedings, in the course of a short conver- 

 sation with Prof. Thompson, and in reply to a question from 

 myself, he stated that he had observed that the resistance of a 

 wire was altered by being coiled. Mr. Latimer Clark also 

 observed that he had noticed the change, but added that the 

 changes were not always constant, giving sometimes an 

 increase and sometimes a decrease in the resistance. Now 

 experiments had given me the same results, and had the 

 subject cropped up during the discussion, I had prepared a 

 few notes to give to the meeting. 



The fact of having obtained sometimes + and sometimes 

 — readings had puzzled me much, but after the conversation 

 above alluded to I resolved to continue my experiments; and 

 although these are yet far from complete, I have felt it my 

 duty to lay the results already obtained before the Society. 



Before going into a description of the apparatus and method 

 employed (part of the apparatus is before you), I would crave 

 a few moments to allude to some of my earlier experiments. 

 These were conducted in a very primitive way ; but the results 

 obtained, although appearing so erratic and contradictory, 

 have been in a great measure confirmed by using the more 

 elaborate method employed later on. In confirmation of this 

 I would draw your attention to the great resemblance which 

 exists between the two curves on the diagram, which are the 

 curves of copper wires produced by hand- and machine- 

 winding respectively. 



The method first employed to investigate this effect was to 

 let the wire under test form one side of a Wheatstone bridge, 

 doubling the wire into a loop as in a resistance-coil, and rolling 

 it by hand on to a cylinder or other shapes in wood. The 

 resistance could thus be taken after each operation without the 

 necessity of altering the connexions. 



The time lost in waiting until the heat imparted to the wire 

 through handling had disappeared was great, and the uncer- 

 tainty as to the tension with which the wire was coiled and 

 uncoiled caused me to seek some other method. 



After many schemes the one before you was adopted. Bear- 

 ing in mind that many of the wires to be experimented upon 

 would not be insulated, and would be of many different dia- 

 meters and rigidities, it was very early evident that the drum 

 on which the wires were to be coiled would also have to insu- 

 late each convolution from its neighbour as well as to wind up 



