IX. On a new Resistance Thermometer. | To Professor John Tyndall, F. R. S. &c., Royal Institution.
3 Great George Street, Westminster, S. W. | December, 1860.
My DEAR SIR,
YOU 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,2 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 spontaneous 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) units3 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, into 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 opposite directions by the current of the battery. One circuit is completed by the insulated thermometer coil, and the other by a variable resistance coil of German silver wire. Instead of the differential galvanometer, a regular Wheatstone’s bridge arrangement may be employed.
You will readily perceive that if the thermometer coil before described were placed in snow and water, and the variable resistance coil were stoppered so as to present 1000 units of resistance, 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 water should rise, say 1° Fahr., its resistance would undergo an increase of 1000 x ·0021 = 2.1 units of resistance, necessitating an addition of 2·1 units to the variable resistance coil in 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 temperature 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, connecting 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 looker-on respecting 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. Wm. Siemens.
C. W. Siemens, ‘On a new Resistance Thermometer’, Phil. Mag., 21 (1861), pp. 73–4.
Charles William Siemens: Charles William Siemens (1823–83). Born Carl Wilhelm Siemens, he was an engineer and businessman who worked in Britain for much of his life and became a British subject. He was elected a FRS in 1862.
the Rangoon and Singapore telegraph cable: In the autumn of 1860, the Siemens Company was contracted to superintend the making and laying of the cable between Rangoon and Singapore: McDonald, Donald & Hunt, Leslie B. A History of Platinum and its Allied Metals. (London: John Matthey, 1982), pp. 354–5. First intended by the British Government for a telegraph connection from Falmouth to Gibraltar, the cable was temporarily reassigned for a connection between Rangoon to Singapore, before finally being used for the Malta to Alexandria Telegraph in May of 1861. It was produced by the Gutta-Percha Company: John Timbs, The Year-Book of Facts in Science and Art (London: Lockwood and Co., 1862), p. 177.
(Siemens) units: Fearing that the current resistance of metal wires was amendable by their transport, use and varying impurities of the metal, Siemens believed that the resistance of such wires could not reliably be used to define a Standard Measure of current resistance. Instead, he proposed an independent Standard Unit of resistance based on the resistance of mercury, as this metal was easily procurable, rendered pure and its resistance was more independent of temperature than alternative metals: M. W. Siemens, ‘Proposal for a new reproducible Standard Measure of Resistance to Galvanic Currents’, The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science, 4th series, 21 (1861), pp. 25–38. It is not to be confused with the Siemens Unit used today, which was named in honour of Siemens, but is otherwise unrelated to his previous Standard.
Please cite as “Tyndall1755,” in Ɛpsilon: The John Tyndall Collection accessed on