AND STRAIN ON THE ACTION OP PHYSICAL FORCES. 
57 
largely as the specific gravity increased; in fact, the value of e is roughly proportional 
to A 8 . 
Beetz* has also determined the values of e for several carbon rods, used for electric 
lighting, by longitudinal vibrations, and it would appear from the results obtained by 
him that e-j-A 8 is also fairly constant. The mean values of Beetz’ results and my 
own are given in the next table. 
Table III. 
Specific gravity =A. 
Value of e in grammes 
per square centim. 
e-f- A s . 
Beetz. 
1-532 
152-0 xlO 6 
501x10 4 
1-547 
154-7 „ 
466 ,, 
1-564 
174-7 „ 
488 „ 
1-580 
193-5 „ 
498 „ 
1-593 
205-4 „ 
495 „ 
1-631 
254-8 „ 
509 „ 
Mean value of e- 
i-A 8 =493 x 10 1 
Tomlinson. 
1-518 
233-8 x 10 6 
829 x10 l 
1-550 
264-3 „ 
/93 ,, 
1-580 
304-2 „ 
783 „ 
Mean value of e- 
■^A s =802 x 10 4 
Though, however, e-r A 8 is roughly a constant for different specimens of carbon by 
the same maker, the value of this constant may he evidently very different for samples 
from different makers, the mean value of Beetz’ samples for e-f-A 8 being 493 X 10 4 
and that of my own specimens 802 X 10 4 . 
The value of e given in Table II. for number 3 is calculated from the formula 
e-j-A 8 =802 X 10 4 . 
It will be noticed that the product eXx, i.e., the increase of resistance which would 
be caused by a load sufficient to double the length of the rod, is nearly a constant for 
the different specimens, and on referring to Table I. it will be observed that this product 
is much less than that for several of the metals, though the increase of resistance caused 
by a stress of 1 grm. per square centimetre is of course, in consequence of the small 
elasticity of the carbon, greater than any of the metals examined except tin and lead, 
whose elasticities approach much more nearly to that of this kind of carbon than any 
of the other substances.! 
In calculating the alteration of specific resistance it was assumed that a— '250, but 
* Ann. der Phys. und Cliem., 1881, No. 1, p. 67. 
f Tlie elasticity of the rods here examined is very nearly equal to that of tin. 
MDCCCLXXXIII. I 
