44 REPORT — 1861. 



There can therefore he no douht of the existence of a connexion between the 

 cycles in the mean yearly values of the magnetic elenients, and the disturbance 

 cycle. It is worthy of remark, however, that the epochs of greatest and least 

 declination precede those of greatest and least distiu'bance, while the corresponding 

 epochs for the horizontal intensity /o/fo«« them. 



On an Electric Resistance Thermometer for ohserving Temperatures at inacces~ 

 sible situations. Bij C. "W. Siemens. 



The Philosophical Magazine for January 1861 describes a method which I had 

 had occasion to resort to for ascertaining the temperatui-e of the interior of a mass 

 of electi'ic telegraph cable suspected of spontaneous generation of heat. Coils of 

 copper of platinum wire, of known resistances, were placed between the layers of 

 cable while coiling it, and leading wires conducted to an observatory. The tem- 

 perature of the cable could at any time be ascertained by measiu-ing the actual 

 resistances of these coils by means of a Wheatstone's bridge arrangement, and 

 comparing the residts with the resistances of the same coils at a standard tempe- 

 rature. The electric resistance of a copper or well-annealed platinimi wire, increasing 

 in a very uniform ratio with increase of temperatiu-e, enabled me to detei-mine the 

 latter with a remarkable degree of accui-acy. 



In endeavouring to simplify the arrangement, I have succeeded in dispensing 

 entirely with the Wheatstone's bridge, and, in fact, in reducing the observation to 

 the mere reading of an ordinary mercm'y thermometer. 



The apparatus consists of a differential galvanometer, and of a bath of water or 

 oil, the temperature of which can be changed at will by opening one or other of 

 two cocks, one supplying cold and the other hot liquid, an ovei-flow pipe being 

 provided to prevent accmnidation. A battery of from fom- to eight cells is pro- 

 vided, besides a nimiber of coils, each consisting of a certain length of thin insidated 

 platinum wire enclosed in a sealed metal tube. These coils ha^^ng been carefuUy 

 adjusted, in the first instance, to ofier an equal resistance at a fixed temperature, 

 are connected with insulated copper leading wires, of comparatively large sectional 

 arcs, the ends of which are brought to the binding screws of the apparatus, to be 

 inserted, when required, in a circuit including the batteiy and one side of the difier- 

 ential galvanometer. 



These "thermometer coils" are deposited at the places whose temperatiu*es have 

 to be observed, excepting one which is reserved for comparison with the others. 

 This last-mentioned coil, connected by means of its leading wires so as to form an 

 electxic cu'ctiit with the battery and the other side of the differential galvanometer, 

 is immersed in the bath before mentioned. This latter coil is enclosed in a sealed 

 auricular chamber formed by an internal and external tube of copper or brass, which 

 on being immersed immediately communicates the temperature of the bath to the 

 coil. 



It is evident that if the temperature of the bath be the same as that of the place 

 where the thermometer coil under examination is deposited, the divided battery 

 currents will meet, on each side, an equal resistance, and passing through the two 

 helices of the difierential galvanometer in opposite directions, will produce no visible 

 effect upon the needle. 



If, however, the temperatures of these coils shoidd be unequal, the needle will be 

 deflected by the preponderance of cmi-ent in the cooler half of the divided circuit, 

 showing by the direction of its deflection whether cold or hot liquid should be 

 added to the bath to establish equilibrium of ciurents. 



When this equilibrium is obtained, the temperature of the bath is observed by 

 means of an ordinary mercury thermometer, and must necessarily be identical with 

 the temperatm-e at the distant stations where the coil under examination is de- 

 posited. 



By di\dding the thermometer coils into two portions, the apparatus is rendered 

 applicable for observing wider ranges of temperatine than can be attained directly 

 by the mercuiy thermometer, and in this modified form it may be used for pyrome- 

 trical purposes. 



The employment of equal and undivided coils in measuring ordinary tempe- 

 ratures is, however, no^ only the more simple aiTangement but it has the advan- 



