236 



NATURE 



\ytily 22, 1875 



up to the boiling-point of the latter metal, results at higher tem- 

 peratures were obtained. The first part concludes with several 

 pages of tabulated results of experimentF, which results are laid 

 down in a sheet of diagrams. 



In the second part, Dr. Siemens describes the instruments he 

 has designed for the measurement of temperature by electrical 

 resistance, having first referred to the coils of silk-covered copper 

 wire, by which he was enabled to detect a dangerous rise of tem- 

 perature in the Malta and Alexandria Telegraph cable, coiled in 

 ship's hold, and saved that cable from being destroyed. The 

 simplest of these arrangements is shown in Fig. 2, and is em- 

 ployed for the measurement of temperature not exceeding 

 the boiling point of water. Insulated wire is wound round a 

 cylindrical piece of wood and is enclosed in a metal casing : one 

 end is joined to a thicker insulated wire, and the other to a 

 similar one soldered to it ; this is called the thermometric 

 resistance coil or thermometer coil. The thermometrical com- 

 parison coil is formed of an exactly similar wire, and has an 

 equal resistance with the other. The wire is wound upon a 

 metal tube, and is enclosed in a protecting capsule of metal, in 



the open end of which is fitted a vulcanite stopper through which 

 are passed the leading wires attached to the coil. This is placed 

 in a movable tube having a flanged bottom and containing a 

 mercury thermometer; the tube is immersed in a cylindrical 

 vessel of water, wherein it is moved up and down, the flange 

 agitating and thus equalising the temperature of the water. 

 The thermometer coil, which may be at a distance from the 

 place of observation, is connected with the comparison coil 

 through a pair of equal resistances and a galvanometer. "When 

 electrical equilibrium is obtained, by adding hot or cold water to 

 the vessel containing the comparison coil until the galvanometer 



needle is at the zero'of its scale, 



A T +1 

 it is evident that jj —'~7i~]i 



A and B representing the equal resistances, / and /' the equal 

 resistances of the leading wires, and T t' those of the thermo- 

 meter and resistance coils, or T = t', and the temperature of the 

 water in which the comparison coil is placed will be that of the 

 distant station. 



In measuring deep-sea temperatures the coil must be so pro- 

 tected as to be perfectly insulated at the greatest depths, and the 



Fig. 3. 



wire so coiled as to be effected by slight variations of temperature 

 in its vicinity. The necessary instrument is shown in the 

 sketch Fig. 3, which represents an insulated wire coiled on a 

 metal tube ; ope end of the wire is soldered to the tube, the other 

 to a copper wire insulated with gutta percha, and carried through 

 a hole to the interior : over each end of the tube is drawn a piece 

 of vulcanised india-rubber pipe, and over the whole a larger 

 piece of india-rubber tubing, which, after being padded outside 

 with hemp yarn, is lashed tightly with a stout binding wire. 

 The gutta-percha covered wire is placed between the india-rubber 

 pipes b and c, its end being soldered to one of the leading wires, 

 the other leading wire being soldered to the brass tube. The 

 whole is carried at the end of the sounding line, which contains 

 the leading wires. These coils are tested under hydrostatic 

 pressure, and accurate readings are obtained tO a tenth of a 

 degree Fahrenheit. 



The only difficulty that has hitherto arisen in tb© employment 

 of this instrument has been the obtaining of skilled observers to 

 note with accuracy the indications of the galvanoscope on board 

 ship. 



The next instrument described is the electrical pyrometer, the 



coil of which is made of platinum wire, wound on a hard baked 

 pipe-clay cylinder in which a doubled threaded helical groove is 

 formed, and which is shown in Fig. 4. 



At each end of the spiral portion B B, it is provided with a 

 ring-formed projecting rim c and c', the purpose of which is to 

 keep the cylinder in place when it is inserted in the outer metal 

 case, and to prevent the possibility of contact between the case 

 and the platinum wire. Through the lower ring c' are the small 

 holes b b', and through the upper portion two others, a a. The 

 use of the upper holes a a' is for passing the ends of the platinum 

 wires through, before connecting them with the leading wires. 

 From these two holes downwards platinum wires are coiled in 

 parallel convolutions round the cylinder to the bottom, where 

 they are passed separately through the holes, bb'. Here they 

 are twisted, and by preference fused together by means of an 

 oxyhydrogen blow-pipe. At this end also the effective length 

 and resistance of the platinum wire can be adjusted, which is 

 accomplished by forming a return loop of the wire, and providing 

 a connecting screw-link of platinum, L, by which any portion of 

 the loop can be cut off from the electrical circuit. 



The pipe-clay cylinder is inserted in the lower portion of the 



protecting case seen in Fig. 6. This part of the case is made of 

 iron or platinum, and is fitted into the long tube, which is of 

 wrought iron, and serves as a handle. "When the lower end of 

 the casing is of iron, there is a platinum shield to protect the coil 

 on the pipe-clay cylinder. The purpose of the platinum casing 

 is to shield the resistance wire against hot gases, and against 

 accident. At the points A A, Fig. 4, the thick platinum wires 

 are joined to copper connections, over which pieced of ordinary 

 clay tobacco-pipe are drawn, and which terminate in binding 

 screws fitted to a block of pipe-clay, closing the end of the tube. 

 A third binding screw is provided, which is likewise connected 

 with one of the two copper connecting wires, and which serves 

 to eliminate disturbing resistances in the leading wires. The 

 pipe-clay cylinder is, when cold, highly insulating ; its conducting 

 power increases with heat, but not to an extent to produce error, 

 as the variation is inappreciable until a white heat is reached, 

 but in measuring temperatures above a white heat, the indica- 

 tions of the instrument are slightly below the true value. In 

 measuring temperatures with this instrument the differential 

 voltameter is employed, a wide range of resistances being 



obtained ; this instrument forms the subject of the third part o 

 this paper, to which we now refer. The theory of differential mea 

 surement and the instrument employed are thus described by 

 Dr. Siemens : — 



Faraday established the law that the decomposition of water 

 in a voltameter in an unit of time is a measure of the intensity cf 

 the current employed ; or, that 



/ = ^; 

 t 



— /being the intensity, Fthe volume, and t the time. 



According to Ohm's general law, the intensity, /, is directly 

 governed by the electro-motive force, E, and, inversely, by the 

 resistance. A', of the electric circuit, or, it is 



Combining the two laws we have 



J? 

 which formula would enable us to determine any unknowr>. 



