DISCHARGE OF ELECTRICITY FROM HOT PLATINUM. 
249 
The resistance of 0° C. was calculated from the resistance at the temperature of the 
room, using the known value (0’0038) of the temperature coefficient at low 
temperatures of the wire used. The loop and wires D and E were made from pure 
platinum supplied by Messrs. Johnson, Matthey and Co. 
The temperature of the loop was deduced from its resistance in the following 
manner. The temperature coefficient of the wire used was determined by measuring 
the resistance of a piece of the wire cold and at the melting-point of pure KoSO^^, 
which, according to Heycock and Neville, is 1066° C. The wire was hung 
horizontally, and screened from air currents, and a minute particle of KgSO,^, put on 
it and observed through a microscope. It was then heated by passing a current 
through it until the particle of K^SO^ just began to melt, and its resistance when this 
occurred was measured by the bridge arrangement just described. This was done 
with a particle at several points along the wire, and the mean of the resistances when 
the particles melted, which were all very nearly the same, was taken to be the 
resistance at 1066° C. 
The following are the results of one experiment of this kind ;— 
Resistance in variable arm of bridge required to balance resistance of 18 centims. 
of wire, 0*2 millim. in diameter, at 19° C., 1014 ohms. 
Resistances required at 1066° C. : (1) 4098, (2) 4100, (3) 4102; Mean 4100 ohms. 
Ptesistance at 0° C. = 
10 14 
1+19x00038 
= 9457. 
Hence mean temperature coefficient of the wire between 0° C. and 1066° C. is 
,,, Vf,: - = 0'003129 ohm per ohm at 0° C. 
The “ platinum temperature ” of the wire loop in any experiment was first calculated 
from its resistances at 0° C. and at the required temperature by means of the formula 
R = Ro(l oLpt), where Rq is the resistance at 0° C., R that at the required 
temperature {pt), and a the coefficient 0'003129. The “ jilatinum temperatures” so 
determined were corrected to the centigrade scale liy means of a table of corrections. 
These corrections were obtained by determining the resistance of the wire at tlie 
melting-point of platinum, which was taken to be 1820° C. This was done by heating 
a wire loop in the apparatus described above until it just melted. The wire was 
heated in a vacuum and the current increased (very gradually vdien close to the 
melting-point), and the resistance measured until the wire just melted. This method 
gave sufficiently concordant results. The following are the numbers obtained in two 
experiments of this kind :—■ 
(1) Balancing resistance at 15° C. for wire OH millim. in diameter and about 
13 centims. long, 1320 ohms. 
Ditto when just on point of melting, 7615 ohms. 
YOL. COIL-A. 2 K 
