in Electric Resistance produced in Wires by Stretching. 533 



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 AVheatstone- 

 bridge arrangement. 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 intended to be used 

 were first put on and then taken off. Afterwards the wire was 

 balanced as nearly as possible by German-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, 

 w 7 hich 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 off 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 4 pianoforte steel wires, 1 wire of commercial 

 steel, 3 iron wires, and 4 brass wires were examined with several 

 different weights. The wires taken were of various sections, and 

 it was found that in each case the increase of resistance was 

 " exactly proportional to the stretching force," the stretching not 

 being carried beyond the limit of elasticity of each wire. The 

 resistance of a cubic centimetre of each wire was then determined, 

 also the increase of resistance which a cubic centimetre of each 

 wire would experience when stretched by a force of 1 gramme 

 in the same direction as the passage of the current was calcu- 

 lated from the observations made. The former values varied from 



1574-8 x 10" 8 to 1882-4 x 10~ 8 in tne case o£ steel > from 

 1200-8 x 10~ 8 to 1291-0 x 10~ 8 in the case of iron, and from 

 656-7 X 10" 8 to 782-2 x 10 -8 in the case of brass ; 

 the latter values varied from 



2982 x 10~ 17 to 3511 x 10" 17 in the case of steel, from 



27 —17 



2557 X 10 to 2712 x 10 in the case of iron, and from 

 1565 x 10" 17 to 1843 x 10~ 17 in the case of brass, 

 the numbers in each case representing so many ohms. 



