350 Proceedings of Royal Society of Edinburgh. [sess. 
consisted of seven strands. The four leads used for joining the coils- 
with the bridge were placed in the positions which they actually 
occupied during the conductivity measurements, and their resist- 
ances were determined both individually and collectively by a fall 
of potential method. The several results were very concordant, and 
gave a mean value of *0296 true ohm at the ordinary temperature 
of the laboratory. This result was checked by calculating the 
value of the resistance from the known dimensions of the leads 
and the generally accepted value for the specific resistance of 
copper in the form of wire ; the number deduced differed only 
very slightly from that obtained experimentally. 
The temperature of the laboratory varied but little during the 
time required for the whole of the conductivity measurements 
hence the change in the resistance of the leads due to the variations 
in the temperature of the room would be insignificant and 
inappreciable. 
The Electric Current and Telephone . 
The necessary alternating current was obtained by using a type 
of small induction coil enclosed in a sound proof box, and made 
by Fritz Kohler of Leipzig, and a sodium bichromate battery 
consisting of three 1 -quart cells arranged in series, and each having,, 
when newly made up, an E.M.F. of about 2-3 volts. 
For detecting the balancing point upon the bridge wire, a 2-pole 
telephone of French make was found to be exceedingly sensitive. 
The Electric Conductivity Cell. 
The cells which are so largely used for the determination of 
relative and molecular conductivities are ill adapted for accurate 
measurements. One of the chief difficulties with which one has to 
contend when using them, arises from the evaporation of a portion 
of the solvent into the relatively large air-space immediately 
above ; not infrequently a condensation of the vapour follows, 
whereby drops are formed upon the upper portions of the cell 
from time to time these will trickle down into the main body of 
the liquid, and suddenly disturb a balance that may have been, 
obtained ; in consequence very uncertain data are procured for the 
calculation of the desired conductivity. The difficulty may be 
