248 
DR, HAROl.D A. WTT.ROX ON TTTE 
titted it loosely and was welded to two platinnin wires sealed tliroiiii’li the end of tlie 
t\d)e at F. 'I’lie loop was welded to two platinum wires 1) and E, wliicli were sealed 
tlironodi the other end of the tube as shown. These wires were each 1 millim. thick 
and 2 centiins. long, and the current used to heat tlie loop entered tlirough tliein. 
'fhe cylinder was connected through a reversing commutator to an Ayrton-Mather 
galvanometer 0^, and tlien through a liquid resistance of about 10'’ ohms and another 
Commutator to one pole of a battery of small secondai^ cells, the othei pole 
of whicli was connected to the loop. The P.D. between tlie loop and cylinder was 
measured liy means of an electrostatic voltmeter \ . 
The resistance of the loop was measured liy means of a Wheatstone's bridge 
arrangement. 'The loo}) was connected in series with a resistance coil of thick 
German silver wire spirals immersed in a jar of })arafHn oil, the temperature of which 
was indicated by a normal thermometer. 
The coil Pi and tlie loop formed two arms of the bridge, and foi' the other two 
arms tlie coils of a P.O. box were used. The coil P] had a resistance of about 
1 -3 ohms, and 1000 or 200 ohms was usually used for the adjacent arm in the box. 
The fourth arm then had a value, usually several tliousand ohms, depending on the 
resistance of the loop. The current through the loop, which nearly all passed through 
the coll P^, was supplied by a battery of fi’om 4 to 10 secondary cells, and was 
measured by means of a A^eston ammeter. Another Ayrton-Mather galvanometer 
was used with the bridge arrangement, and was connected to the wire E and tlie 
o]j])Osite corner of the bridge. A key was used with this galvanometer, vhich ciosed 
fir-st the two arms of the bridge in tlie P.O. box and then the galvanometer circuit. 
In this way the current only passed through the coils in the box when the key was 
pressed down. The resistance of these cods was so large compared vith that of Pj 
and the loop tliat no appreciable change in the current through the loop occurred 
when the key was pressed down. A rlieostat in the battery circuit enabled the 
current through the loop to he varied, and the method used was to give the resistance 
of the varialile arm of the bridge the desired value, and then by altering the current 
through the loop by means of the rheostat balance the bridge. At high temperatures 
the liridge was very sensitive and the resistance of the loop could easily lie kept 
constant to one part in 5000 by adjusting tbe rheostat. M hen it was desired to 
measure the resistance of the loop cold, a single Leclanche cell in series with a 
resistance of 10 ohms was substituted for the secondary cells, and the variable arm of 
the bridge adjusted till a balance was obtained. This measurement could he made to 
one part in a thousand. The temperature of the loop was got in this case by a 
thermometer placed close to the tube, a,nd whenever possible this measurement was 
made with the apparatus filled with gas at atmos})heric pressure, because at low 
jiressures a very small current is sufficient to heat the loop appreciably. Ihe cm lent 
tlirough the loop in this case, therefore, was only ke})t on for a few seconds at a time, 
in order to prevent the loop from being lieated above the surrounding tempeiatuie. 
