452 RECENT PROGRESS IN PHYSICS- 



If an electrical thermometer besides the thin wire he introduced into 

 the circuit, lengthening the wire will also occasion a less heat in the 

 thermometer by retarding the discharge. 



Since the " force of the current" is measured by the temperature of 

 the electrical thermometer, it also may be said that the force of the 

 current is diminished by the prolongation of the thin wire. 



If, then, a certain charge of the battery brings the wire to ignition, 

 by lengthening the wire, the same charge will yield a current of less 

 force, and it will no longer be sufficient to jjroduce incandescence in 

 the wire. To make the longer wire glow, the charge must be in- 

 creased, as shown by Bless' experiments, until the force of the current 

 has reached its previous magnitude. 



A platinum wire 15.7 lines long was brought to incandescence by 

 four jars and a quantity of E 12, the indication of the thermometer 

 being 8. 



An equally thick wire, 77.5 lines long, was brought to incandes- 

 cence by four jars and a quantity of E 22, the indication of the ther- 

 mometer being likewise 8. Wires equally thick, but of different 

 lengths, were, therefore, brought to ignition by currents of the same 

 force. 



3. Ignition of wires in proportion to their thickness. — If a given force 

 of current produces ignition in a wire, with an equal value oi q and s, a 

 thicker wire of the same length will not produce that effect, although 

 the force of the discharge current increases on account of the dimin- 

 ished retardation. 



To produce incandescence in thick wires, q must be increased, by 

 which the force of the current is also increased. 

 _ For wires of equal length, with radii of 0.018 in., 0.021 in., 0.026 

 lines, respectively, discharge currents were required whose forces, 

 measured in the electrical thermometer, were 9, 20, 43. 



The fourth powers of the three radii are to each other as 10 : 



19 : 45, and these numbers are nearly in the same proportion as 9 : 



20 : 43. Hence, 



The force of the discharge of an electrical battery, necessary for pro- 

 ducing ignition in a wire, is p)ropo7^tional to the fourth power of the radius 

 of the ioi7~e. 



4. Ignition of wires of different metals. — It follows from the experi- 

 ments that Riess made on the ignition of wires of different metals, 

 that if 1 indicate the force of the current required to produce ignition 

 in a platinum wire, wires of the same dimensions, consisting of the 

 following metals, are brought to the same condition by currents as 

 follows : 



