TEMPEEATIJEE ON THE ELECTEIC CONDUCTINO POWEE OF METALS. 
generally worked with only one of the commutators, and therefore mostly used the one 
half of the wire, the zero-point of the wire only varied, during the whole of the experi- 
ments, which have taken almost a year to carry out, 3 milhms. The zero-point was 
always determined before each series was begun. The distance the block M was moved 
when the resistance of a wire was determined, first at 0° and then at 100°, was, for pure 
metals in a solid state, about 800 millims., or about 8 millims. for 1°. As, however, the 
movement of the block M of 1 millim. caused a deflection of the needles of the galva- 
nometer I of 20° to 30°, it is evident, with the apparatus employed, that the differences 
in the resistance of a wire to values less than those corresponding to 0°T C. can be 
accurately determined. Our results, moreover, prove this to be the case, as in many 
instances the difference between the observed and calculated conducting powers for the 
whole series do not amount to values equal to 0°T to 0°’2 C. 
The trough B is a double one, the space between the inner and outer one being 20 
miUims. The dimensions of the inner trough were 400 millims. long, 80 millims. -wide, 
and 80 millims. deep. Through the ends of both two holes of about 20 millims. wide 
were made, in which good corks were fitted, and through these passed the thick copper 
wires F ; and also at one end a glass tube fZ, wide enough to allow the thermometer c 
to pass ffeely. A piece of india-rubber tubing, fitting over the glass tube d, and tightly 
round the thermometer, closed the tube, but allowed the thermometer to be moved either 
backward or forward with great ease. The tubes a are for filhng the space between the 
inner and outer troughs with oil. 
The wire to be tested lay in the trough, as shown in fig. 2, on a small glass tray, 
made by sphtting a glass tube longitudinally, thereby preventing any possibility of its 
touching the trough, and also guarding it from being moved by the stirrer. A second 
trough, of somewhat smaller dimensions, was also used. 
The use of an oil-bath for heating the wires has been objected to by a former 
observer* ; it was therefore necessary to determine experimentally whether there was 
any real reason for the objection or not. He states that, as oil conducts electricity 
better on being heated than when cold, the differences between the conducting powers 
of cold and hot oil will materially affect the values obtained for the resistances of wire 
which had been determined at different temperatures in that liquid. In order to test 
the accuracy of this assertion, two copper plates of about 150 millims. diameter were 
connected, the one with the galvanometer, the other with a single Bunsen’s cell ; and to 
complete the circuit, this was connected ■v\nth the galvanometer. A piece of filtering- 
paper, moistened with the ohve-oil used, was placed between the copper plates, and 
these were pressed together with a weight. On completing the circuit not the slightest 
deflection of the needles was observed; the copper plates were then heated to above 
100° C., and still no deflection was visible. To show that the connexions were good, a 
drop of water was put on the oiled paper ; and immediately the needles of the galvano- 
meter were sent with great violence to the stops. This proves that although oil may 
* Abxdsten, Poggexdobff’s ‘ Axmalen,’ vol civ. p. 1. 
B 2 
