MEASUREMENT OE TEMPERATURE. 
197 
oxidation, which would produce the same effect, the wires are sealed up in a hard 
glass or porcelain tube nearly exhausted, from which the air has been removed by 
substituting C0 2 or N a . 
The insulation resistance from one wire to the other can also be measured with the 
same bridge if less than a megohm. It may be neglected if it is greater than 
100,000 ohms. This method affords also a fairly accurate means of measuring the 
temperature variation of the resistance of non-conductors, which begin to conduct 
electrolytically when heated. The chief difficulty is polarisation, but this may be 
avoided in the usual way. 
It would, of course, be possible by this method of comparison to draw up very 
accurate Tables of the resistance variation of metals and other conductors referred to 
platinum as the standard. Such Tables would, however, be of little practical use, 
apart from the particular specimens of wire for which the comparisons were made, 
except in so far as they tended to elucidate the phenomenon itself. A Table 
of empirical formulae of the type Ii/R 0 = l atfit 2 has been published by Benoit for 
most common metals. 
The method of comparison was not adopted, as far as I can discover, in spite of its 
obvious advantages, and as he used different specimens of wire his results do not 
agree very closely with mine. It is, however, from a thermometric point of view that 
this method of comparison is of the greatest possible importance. For this reason we 
proceed to give full details of a comparison between two platinum wires. Since the 
same apparatus was used for all the comparisons it will be described once for all. 
The Diagram of Connections (Plate II, fig. 3) shows the double Wheatstone bridge 
arrangement adopted. 
BO 10 
The arm of the balance, AB, is divided in the ratio ——, = ——the adjustable resis- 
tances, AD, AE, are taken from post-office boxes, and may have any value up to 
10 ,000. The sensitive galvanometer may be put in circuit between D and C, or 
C and E, or D and E, so that the resistances of either DB or BE, or their ratio, may 
be observed without the necessity of constantly altering the resistances in the arms 
AE, AD, except in so far as is necessary to follow the slow change of temperature. 
A complete observation of both resistances can be thus made in a few seconds. 
The actual arrangement of the apparatus is shown in Plan (fig. 4), which is lettered 
to correspond with the diagram. The ends of the wires to be compared are fused on 
to copper tails, and the junctions arranged close side by side at J to avoid the pro¬ 
duction of thermo-electric currents. F represents the rocking commutator in the 
battery circuit to eliminate any residual effects. The copper tails are in turn soldered 
to thick copper rods, passing, air-tight, through an indiarubber cork which fits the 
porcelain tube in which the wires are heated. The copper rods are amalgamated, and 
dip into mercury cups, numbered 1, 2, 3, 4, 5, on the plan. Cup No. 2 is connected 
by a thick copper rod to the binding-screw, B, of the resistance-box. Cups 2 and 3 
