METALLIC WIRES BY ELECTRICITY. 443 
experiment in a more complicated manner*. A metallic rod was 
so placed upon a horizontal plane as to be moveable round one 
of its ends, whilst the other finely-toothed end worked upon a 
cog-wheel furnished with a hand. A strong spring pressed 
against the rod, but was checked in its action by a stretched iron 
wire eight inches in length, fixed upon the rod. When the iron 
wire was brought to a red heat by an electrical discharge, the 
hand indicated a movement of the rod in the direction of the 
acting spring, and therefore showed a lengthening of the iron 
wire, and it appeared that the permanent increase in length was 
less than at the moment incandescence appeared. It is evident 
that this was not a primary electrical action, but a mechanical 
effect upon the red-hot wire. I therefore instituted only one 
experiment of this kind, in which however the wire immediately 
broke, in consequence of my having used either too great a weight 
or a too powerful electrical charge. 
Laws or ELECTRICAL INCANDESCENCE. 
Former Statements. 
We possess no practical observations on the laws which regu- 
late the incandescence of wires by electrical discharges. Several 
philosophers have indeed occupied themselves with the heating 
effects of electricity; but they directed their attention to the 
fusion of the metals, and upon that point their statements are . 
contradictory, nor have they been able to obtain any clear insight 
into the nature of the phenomena. The temperature of fusion 
is by no means adapted for a fixed standard point, as there are 
different degrees of fusion, and, as will be seen below, an elec- 
trical action precedes it, which also occasions the destruction of 
the wires. The great diversity in the statements of those phi- 
losophers arises partly from this cause, partly however from an 
answer having been sought to a question which, generally put, 
is not capable of a definite solution. 
Van Marum}+ charged equally a battery of 135 and one of 225 
jars; he found in three experiments that the length of iron wire 
which could be melted by them was in the proportion of 3 to 5. 
Thus the length of wire melted, the intensity remaining the same, 
would be proportional to the quantity of electricity. The lengths 
of wires of different thicknesses, which were melted by a cer- 
* Elektric. Artifice. Torin, 1772, p. 301. 
+ Beschreibung, &c. Erste Fortsetzung, p. 3. 
ELS be 
