508 
MR. 0. W. RICHARDSON ON THE ELECTRICAL CONDUCTIVITY 
experiments a Thomson galvanometer, of 4058 ohms resistance, which gave a 
deflection of 1 scale division for 7 X 10“^° amj^ere, was nsed, but this was replaced 
later by a less sensitive D’Arsonval galvanometer. The sensitiveness of the 
galvanometer deflections could he lowered at will by means of the shunt resistances, 
Pt 3 and PtQ. 
The resistance of the hot wire was determined by placing it in one arm of a 
Wheatstone’s bridge, the other three arms being the resistances Pt^, Pig, and Pi^. 
The battery Bn, which worked the bridge, also supplied the current necessary to heat 
F. As there was a current of up to I'5 amperes continually running through the 
arm F^, the corresponding arm Pin had to be constructed so as to carry this current 
Earth 
without heating. A German silver rheostat of very thick wire mounted on an iron 
frame and having a resistance of I’Z ohms was found to fulfil the required conditions. 
The arin Pig had a resistance of 1000 ohms, and plugs were taken out of the box 
till a balance was obtained in the galvanometer Go. The maximum current which 
flowed through the arms Pg and Pj. was thus about flowing through P.i 
and F. There was therefore no danger of Pig and P^ being heated by the currents 
used. The adjustable resistance Pij^ served to regulate the heating current. By 
means of the keys, Ko, Kg, the battery could be put on between the two 
resistances Pg and Pi^^ at the one end, and Po and F at the other. In this way the 
currents flowing througli the various arms could be made small, F would not become 
heated, and, by making the proper connections for the galvanometer G.i, the 
resistance of the arm F at the ordinary temperature could be determined. In tlie 
earlier experiments an arrangement was used which enabled the same galvanometer 
to be employed l^oth for the resistance and leakage circuits; but it was found to be 
more convenient to use two galvanometers. 
