AND STRAIN ON THE ACTION OF PHYSICAL FORCES. 
53 
But the change of resistance attending a given amount of expansion caused by rise 
of temperature is, in the case of iron, more than a hundred times that resulting from 
the same amount of expansion produced by mechanical stress; and it is evident that 
with all metals the alteration of electrical conductivity following on alteration of 
temperature is due for the most part to other causes than mere contraction and 
expansion. 
Carbon. 
As it seemed desirable to extend these researches to as many substances as possible, 
some experiments were made with carbon rods such as are used for electric lighting 
purposes. These rods were between 40 and 50 centims. in length, and of different 
diameters, and their moduli of longitudinal elasticity could be readily determined by 
holding them in the centre and rubbing them along their length with a resined glove. 
The note obtained by the longitudinal vibrations, though, of course, very high in 
pitch, was quite clear and distinct, and very concordant results were obtained when 
the same pieces were tried at different times. The pitch of the note was determined 
by the syren, and the following experiment, taken at random out of my note book, 
will suffice to show what accuracy can be attained with this instrument* even with 
notes of very high pitch. 
Experiment IX. 
A carbon rod of length ‘496 metre, rubbed longitudinally by means of a resined 
glove, gave a note, the lower double octave of which was taken on a monochord; the 
syren was then raised to the pitch of the monochord, and the number of vibrations 
counted for two minutes at a time. 
Number of trial. 
Number of vibrations recorded by 
the syren in two minutes. 
1 
6860 x 20 
2 
6822x20 
3 
6835 x 20 
Mean . 
= 6839 x 20 
In this experiment the rnonochord was re-tuned at each trial, and it will be observed 
that the mean value does not differ from any of those forming it by so much as per 
cent. Of course, by extending the time of each trial and the number of observations, 
still greater accuracy could have been obtained. 
The formula employed for calculating the elasticity is E = 
n~ x 4 l 2 x A 
9810 5 
in which 1 is 
* I have to thank Mr. Furse, the Curator of the Physical Museum at King’s College, for his able 
assistance in this part of the work. 
