1876.] On Resistance to the Passage of an Electric Current. 451 



II. On the Increase in Resistance to the Passage of an Electric 

 Current produced on Wires by Stretching.^^ By Herbert 

 ToMLiNSON^ B.A._, Demonstrator of Natural Philosophy^ King^s 

 College,, London. Communicated by Prof. W. G. Adams^ 

 F.E.S. Received November 14, 1876. 



(Abstract.) 



The object of this inquiry was 



(1) To determine the relation between increased resistance to the 

 passage of an electric current and stretching force. 



(2) To ascertain how much of the increased resistance in each case is 

 produced by mere increase of length and diminution of section of the 

 stretched wire. 



In order to determine the increase of resistance from stretching, the 

 wires were each divided into two parts, about 14 ft. in length ; one end 

 of each part was fastened to a stout hook firmly fixed into a block of 

 wood. These two hooks were about 8 inches apart, and the block of 

 wood in which they were fixed was securely fastened across two uprights 

 placed resting against a wall of the room, so that the weights, which 

 were attached to the other ends of the wires, might swing clear of the 

 table. The two parts of the wire were joined at the top, about 2 inches 

 below each hook, by a small piece of copper wire, which was securely 

 soldered on to each part of the wire so as to connect them. Towards 

 the lower extremities of the two parts, about 5 inches above the points 

 of attachment of the weights, two copper wires of small resistance were 

 soldered so as to connect the wires with a Wheatstone-bridge arrange- 

 ment. The increase of resistance was measured by means of a sliding 

 scale of platinum wire divided into millimetre divisions, each equal to 

 •00166 ohm. As the object was to obtain the temporary, and not the 

 permanent, increase of resistance, which permanent increase was found 

 more or less with all the wires, weights slightly heavier than those in- 

 tended to be used were first put on and then taken off. Afterwards the 

 wire was balanced as nearly as possible by Grerman-silver wire without 

 the sliding scale, and then very exactly with the sliding scale, which was 

 connected with one of two resistance-coils of 100 ohms each, which formed 

 the other two sides of the bridge. The weights used were then carefully 

 put on to the wires, and the increase of resistance measured by means 

 of the sliding scale ; the weights were next taken ofE again, and the 

 sliding scale used for balancing once more. If there was any slight 

 difference, as sometimes occurred, between the readings of the sliding 

 scale before the weights were put on and after they were taken off, the 

 mean of the two readings was taken. In order to secure still greater 

 accuracy, as many as eight or ten trials were frequently made with each 

 particular weight, and the mean of all the trials taken. In this manner 



