AND STRAIN ON THE ACTION OF PHYSICAL FORCES. 
169 
Summary of Part II. 
1. The electrical resistances of iron, steel, platinum, German-silver, copper, platinum- 
silver, brass, zinc, silver, aluminium, tin, lead, and carbon are temporarily increased 
by temporary longitudinal stress, the amount of increase being nearly, but not quite, 
proportional to the stress. 
2. The specific electrical resistances of all the above metals, except aluminium, is like¬ 
wise temporarily increased by temporary longitudinal stress ; with aluminium, however, 
the specific resistance is decreased. The total resistance and the specific resistance of 
nickel are both decreased by temporary longitudinal stress not exceeding a certain 
limit, whilst beyond this limit further increase of stress begins to produce increase of 
resistance. The alterations, both increase and decrease, are very considerably greater 
with this metal than those of any of the other substances examined. 
3. The temporary alteration of specific resistance caused by stress is much less with 
the alloys German-silver, platinum-silver, and brass than with the several components 
of these alloys ; this would suggest an apparent relation between the change of resistance 
caused by alteration of temperature and that due to mechanical stress ; the former 
effect, however, is very much greater than the latter, if we regard the alterations of 
resistance attending the same amount of expansion in each case, and there is no doubt 
that the increase of resistance ensuing on rise of temperature is due almost entirely to 
other causes than mere expansion. 
4. The elasticity of carbon rods varies considerably with different specimens, even 
from the same maker, and is nearly proportional in this case to the eighth power of the 
density. Thick rods have generally a less density than thin ones, and less elasticity. 
5. The specific resistance of carbon also varies considerably with different specimens 
from the same maker, but there is no apparent relationship between specific resistance 
and elasticity. 
6. The increase of resistance caused by longitudinal stress is with different specimens 
of carbon as with different specimens of other substances very nearly proportional to 
the amount of temporary elongation produced by the stress, and though with the 
exception of tin and lead, the total alteration of resistance resulting from a given 
amount of stress is less with the metals which have been examined than with carbon, 
this is not so with regard to the alteration of specific resistance. 
7. Compression produces on the electrical resistance of substances an effect of a 
contrary nature to extension. 
8. Stress applied in a direction transverse to that of the current produces both 
temporary and permanent alteration of resistance of a nature opposite to that resulting 
from longitudinal traction. 
9. Stress applied equally in all directions diminishes the total and the specific 
resistance of most metals. 
MDCCCLX XXIII. Z 
