18 
Proceedings of the Royal Society of Edinburgh. [Sess. 
The length of the lead wire under investigation was measured by 
means of a steel scale stretched vertically beside the wire before any 
permanent stretch had been made. For some time previously a small 
weight was allowed to hang on the wire in order to remove any bends. 
The measurement of the increase of length after stretching was made by 
means of a cathetometer, there being a plate-glass window in front of the 
cupboard to admit of this reading. 
As precision measure for the different readings, we took the maximum 
deviation from the mean, divided by the square root of the number of 
readings. In the case of the cross-section this corresponds to about 3 in 
1600, in the total resistance to 1 in 1000, and in the length to 1 in 2000. 
That component of the error in the specific resistance, due to the error in 
the length, we found to be negligible. 
Considerable difficulty was experienced in obtaining good results, owing 
mainly to the lack of uniformity in some of the wires when stretched. At 
first also we did not make so many readings of the diameter along the 
wire, and the error in the section due to this was large. 
The results for the stretching of the wire which we considered most 
reliable are given in the following table. The first three columns give the 
length, cross-section, and total resistance of the lead wire, and the fourth 
column gives the specific resistance as computed from these values. 
Length in Cms. 
196*2 
204*4 
213*4 
221*4 
Section in Sq. Cms. 
*01688 
*01622 
*01555 
*01493 
Resistance in Ohms. Sp. Res. 
*2207 18*99 xlO" 6 
*2395 19*01 xlO- 6 
*2592 18*93 xl0“ 6 
*2812 18*96 xlO- 6 
The precision measure of the specific resistance, computed from the 
precision measures of the length, cross-section, and total resistance, is equal 
to *04x10“ 6 The above values of the specific resistance show no change 
greater than can be accounted for by the errors in the section and total 
resistance. 
Tomlinson * has studied permanent change of specific resistance produced 
by stretching. He determined the value of the section by finding the 
specific gravity at the beginning and end, and assuming the change of 
section at intermediate stages to be proportional to increase of length. 
The percentage increase of specific resistance per unit of percentage 
increase of length of wire is, for lead, by the above results, not greater 
than ± *017. 
* Phil. Trans., 174, 1, 1883. 
