402 Mr. H. Tomlinson on the Increase in Resistance 



of the wire, and to this the weights were attached by means of strong 

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

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

 on to each part of the wire, so as to connect them ; and toward the 

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

 attachment of the weights, two copper wires were soldered so as to con- 

 nect the wires with a Wheatstone's bridge. The increase of resistance 

 of the wires 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 taken off. Afterwards the wire was balanced as nearly 

 as possible by G-er man- 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 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. This precaution was necessary, as there were continual 

 small variations from slight changes of temperature in the air of the 

 room. These variations of temperature caused at first great trouble in 

 the case of iron and steel wires, as the slightest difference of tempera- 

 ture was at once shown by the shifting of the light on the scale of the 

 Thomson's reflecting galvanometer employed. This instrument was so 

 delicate that the warmth of the hand placed 6 inches from the wire 

 caused a perceptible shifting of the light. It was, indeed, partly owing 

 to this difficulty that the observation of the permanent increase from 

 stretching was not attempted. I have, however, since devised a plan for 

 getting rid of this alteration from changes of temperature, and I hope to 

 be able to make other experiments with wires in which the permanent as 

 well as the temporary increase of resistance will be observed. Most of 

 the observations of the steel and iron wires were made at first in the 

 evening, with the doors and windows shut, as it was found almost im- 

 possible to take very exact observations in the daytime. 



The disposition of the wires is shown in the accompanying sketch. In 

 order to avoid heating the wires by the current, a single Leclanche cell 

 was employed, and an increase of resistance = -5^^- of the whole could 

 easily be measured. 



