531 
of Edinburgh, Session 1885 - 86 . 
instrument especially, I was led to suppose that these currents were 
always present, and that they were subject to more or less regular 
variations, both in direction and intensity. 
A detailed investigation would require more elaborate electrical 
apparatus than can at present be obtained at the Observatory, and 
would interfere with the ordinary use of the wire. The observations 
which form the subject of this communication were made entirely 
by means of a Thomson mirror galvanometer, and were so arranged 
as not to affect in any way the transmission of signals in either 
direction. 
A preliminary trial was first made by putting a coil of copper wire 
in circuit, and placing in the field an ordinary pocket-compass. 
When this was done no observable deflection could be obtained. 
The mirror galvanometer was then put in, and after a few further 
trials the arrangement shown below was adopted, and found to 
give suitable deflections. 
to earth. 
to line. 
In the figure 0 is binding screw leading to wire attached to 
sheath of cable. 
N, binding screw attached to line wire of cable. 
T, telegraph instrument. 
A coil R, consisting of 24 feet of iron wire, No. 17 B.W.G., was 
attached to 0 and N, along with the telegraphic instrument, and in 
multiple arc with this the galvanometer G was inserted. Contact 
was broken at M, except while readings of G were actually being 
taken, so that no part of the current was diverted from the tele- 
graph instrument. As no means of accurately measuring resistance 
was at hand, absolute measurements of the current passing at any 
time could not be obtained, but a rough estimate from tables of 
resistances of iron wire gave R = about 0'4 ohms ; and the resistance 
