1902-3.] Mr Manley on Electrical Conductivity of Sea- Water. 349 
were available for the determination of electric conductivities and 
relative densities. 
The Wheatstone’s Bridge. 
For the determination of the electric conductivities of the 
five samples of sea-water, use was made of a 2-metre bridge 
having a calibrated platinum-iridium wire of 1*5 mm. diameter. 
As the structure of the bridge has been already described in a 
paper communicated to the Royal Society,* it need only be stated 
here that the bridge wire is very highly insulated, that special 
arrangements are made for avoiding thermo-electric currents at the 
juncture of the slider with the bridge wire, and that the index 
line is so placed that the introduction of errors due to parallax in 
reading the position of the slider is practically impossible. By 
calibrating the instrument it was found that the joint resistance of 
the two ends of the bridge was equal to that of 10 mm. of the 
wire ; hence the practical length of the wire is 2010 mm. 
The Resistance Coils. 
The standard resistance coils were made by Messrs Elliot Bros., 
and their values ranged from 0T to 500 true ohms. Mr Griffiths 
•of Cambridge very kindly standardised them before they were 
used in a former investigation,! and a comparison of the coils 
with each other before the work herein described was commenced, 
shewed that the relative values of the individual coils had not 
undergone any sensible change. The temperature of the coils was 
.kept practically constant during any given sets of measurements 
by placing the box containing them within another and consider- 
ably larger one, the space between the two being packed with 
■cotton-wool. 
The Leads and Switchboard. 
The resistance coils were joined with the Wheatstone’s Bridge 
by a special type of switchboard, in which the eight mercury cups 
were highly insulated by mounting them upon pillars of ebonite. 
The connecting leads were made from No. 18 copper wire ; each 
* Phil. Trans., vol. 191, p. 376. 
t Phil. Trans., vol. 191, p. 374. 
