74: C. W. Siemens on a new Resistance Thermometer. 
water should rise, say 1° Fahr., its resistance would undergo an 
increase of 1000 x 002] =2°1 units of resistance, necessitating 
an addition of 2°1 units to the variable resistance coil m order 
to re-establish the equilibrium of the needle. 
The ratio of increase of resistance of copper wire with increase 
of temperature may be regarded as perfectly constant within the 
ordinary limits of temperature ; and being able to appreciate the 
tenth part of a unit in the variable resistance coil employed, I 
have the means of determining with great accuracy the tem- 
perature of the locality where the thermometer resistance coil 
is placed. Such thermometer resistance coils I caused to be 
._placed between the layers of the cable at regular intervals, con- 
necting all of them with the same measuring apparatus in the 
cabin. 
After the cable had been about ten days on board (having left 
a wet tank on the contractors’ works), very marked effects of 
heat resulted from the indications of the thermometer coils 
inserted into the interior of the mass of the cable, although the 
coils nearer the top and bottom surfaces did not show yet any 
remarkable excess over the temperature of the ship’s hold, which 
was at 60° Fahr. The increase of heat in the interior progressed 
steadily at the rate of about 3° Fahr. per day, and having reached 
86° Fahr., the cable would have been inevitably destroyed in the 
course of a few days, if the generation of heat had been allowed 
to continue unchecked. 
Considering the comparatively low temperature of the surface 
of the cable, much incredulity was expressed by lookers-on re- 
specting the trustworthiness of these results; but all doubts 
speedily vanished when large quantities of cold water of 42° 
temperature were pumped upon the cable, and found to issue 72° 
Fahr. at the bottom. 
Resistance thermometers of this description might, I think, 
be used with advantage in a variety of scientific observations,— 
for instance, to determine the temperature of the ground at 
various depths throughout the year, or of the sea at various 
depths, &c. &c. In the construction of this instrument, care 
has to be taken that no sensible amount of heat is generated by 
the galvanic currents in any of the resistances employed. 
By substituting an open coil of platinum wire for the insulated 
copper coil, this instrument would be found useful also as a 
pyrometer. 
But, finding this letter already exceeds its intended limits, I 
shall not enlarge upon these applications, which, no doubt, are 
quite obvious to you. 
I am, dear Sir, 
Yours very truly, 
- C. Wo, SIEMENS. 
