METALLIC WIRES BY ELECTRICITY. 449 
composed of good conducting materials; if the reverse is the 
case, however, the relation may be much more distant. A more 
accurate indication of the lengths of wire rendered incandescent 
cannot be expected from the formula above mentioned, for, as 
will be shown in the sequel, the formulz for heat are not abso- 
lutely accurate, when part of the connecting circuit is in a state 
of incandescence. 
Incandescence of Wires according to their Thickness. 
Three platina wires of increasing thickness were fixed succes- 
sively at the side of the thermometer; I observed with a careful 
increase of charge the first appearance of incandescence in them, 
and at the same time the alteration in the thermometer. 
Experiment 14.— 
+ Radius Change of temperature 
Wire. a eee 
in lines. in thermometer. 
ls 0°0181 8°7 
9°0 
9°4 
SOT S. <2. i970 
2. 0°02089 20°0 
20°0 
20°6 
20°4 . . - 202 
3. 0°0261 42°5 
41°8 
448 . . . 43°0 
The thermometer was changed for one less sensitive, and the 
observation continued with three other platina wires. 
Experiment 15.— 
: Radius Change of temperature 
Ware: in lines. in thermometer. “sient 
3. 0°0261 5°6 
6°0 
6°2 
MGr seas Kone SS 
4, 0°02857 8°0 
8°3 
SO en - 5: bog Bel 
5. 0°04053 31°3 
2 30°8 
310. . . 31°0 
Mean. 
