C. W. Siemens on a new Resistance Thermometer. 73 
is a measure of the velocity of the terrestrial current, that is of 
the total intensity of the magnetic force. According to the 
theory, this force may be supposed to be the resultant of hori- 
zontal and vertical components, and the ratio of the latter to the 
former is the tangent of the Inclination. 
Cambridge Observatory, 
December 22, 1860. 
[To be continued. | 
IX. On anew Resistance Thermometer. 
To Professor John Tyndall, F.R.S., &c., Royal Institution. 
3 Great George Street, Westminster, S.W. 
My pear Sir, December, 1860. 
hehe will probably be interested to hear about a very direct 
application of physical science to a purpose of considerable 
practical importance, which I had lately occasion to make. Having 
charge, for the British Government, of the Rangoon and Singapore 
telegraph cable, in so far as its electrical conditions are concerned, 
I was desirous to know the precise temperature of the coil of 
cable on board ship at different points throughout its mass, 
having been led by previous observations to apprehend spontane- 
ous generation of heat. As it would have been impossible to 
introduce mercury thermometers into the interior of the mass, 
I thought of having recourse to an instrument based upon the 
well-ascertained fact that the conductivity of a copper wire 
increases in a simple ratio inversely with its temperature. The 
instrument consists of a rod or tube of metal about 18 inches 
long, upon which silk-covered copper wire is wound in several 
layers so as to produce a total resistance of, say 1000 (Siemens) 
units at the freezing temperature of water. The wire is covered 
for protection with sheet india-rubber, inserted into a tube and 
hermetically sealed. The two ends of the coil of wire are brought, 
by means of insulated conducting wires, imto the observatory, 
where they are connected to measuring apparatus, consisting of 
a battery, galvanometer, and variable resistance coil. The 
galvanometer employed has two sets of coils, traversed in oppo- 
site directions by the current of the battery. One circuit is 
completed by the insulated thermometer coil, and the other bya 
variable resistance coil of German silver wire. Instead of the 
differential galvanometer, a regular Wheatstone’s bridge arrange- 
ment may be employed. 
You will readily perceive that if the thermometer coil before 
described were placed in snow and water, and the variable re- 
sistance coil were stoppered so as to present 1000 units of resist- 
ance, the currents passing through both coils of the differential 
galvanometer would equal one another, and produce, therefore, 
no deflection of the needle. If, however, the temperature of the 
