Positive Electricity from Hot Bodies. 635 



piece, c (3 cm. in length), of platinum wire exactly similar 

 to p. This shorter piece of wire had also leads like those 



of p, and served to compensate for their resistance. The 

 wire was of pure platinum supplied by Messrs. Johnson & 

 Matthey. The other arms of the bridge consisted of two 

 resistances, A A, each of 5000 ohms. A battery, b, of 10 

 to 12 accumulators supplied the current, which could be 

 regulated by means of a variable resistance, r. Before a 

 measurement of the leak from the platinum was carried out, 

 the ad justable resistance, K, was given a suitable value. The 

 current was then started and the resistance, r, reduced till 

 the galvanometer, g, indicated no current. In this way the 

 wire, p, was given a definite temperature, namely that at 

 which the resistance of a definite portion of it was equal 

 to R. The arrangement thus served to heat the wire, p, and 

 to determine its temperature. 



In fig. 2 (p. 636) is shown the way in which the platinum 

 wires, p and c, were connected to the thick copper wires, ww, 

 conveying the current, and the arrangement for measuring the 

 " thermionic" current. The wire, p, was connected to thick 

 platinum leads fused through the ends of glass tubes, L L. 

 The wires, wiv, dipped into mercury which covered the 

 platinum leads. The whole was fixed by means of a tightly 

 fitting indiarubber stopper, g, in a test-tube shaped vessel 

 provided with side tubes, i t, by means of which dry air could 

 be supplied to it. Surrounding the platinum wire was an 

 aluminium cylinder, D, 2 cm. in diameter, supported at the 

 end of a straight copper wire which was insulated from the 

 rest of the vessel by means of sulphur, S, and the other end 

 of which dipped into mercury contained in a cup, M, in a 

 paraffin block. One terminal of a delicate moving coil- 

 galvanometer, G- (1 scale-division = 6*32 X 10~ 10 ampere) was 

 connected to the negative pole of a battery, B, of .100 volts 



2 T 2 



