A/A' WILLIAM SIEMENS, F.R.S. l6l 



i art 1 1. In the testing room the leading wire, 1', is connected 

 directly with the resistance, B, and with the galvanometer ; whilst 

 1 is c iniiected with the resistance A, the galvanometer and the 

 balance thermometer, T. The leading wires, 1 and 1', arc, of 

 oi]|x.'r, of the same gauge, insulated with gutta-percha and spun 

 up together, so that they are equally affected by changes of tem- 

 perature at intermediate places, and have therefore always equal 

 mees. For protection against mechanical injury, the leading 

 wiivs are covered with hemp and sheathed with a laminated cover- 

 ing of copper. 



Thus arranged, the balance thermometer, T, is immersed in a 

 bath of water, the temperature of which can be varied. 



AViien electrical equilibrium is to ta obtained, it is evident that 



A T 4-1 



the relation = - , must first be established. And since A = B 

 B T +1 



and 1 = 1', it follows that this equilibrium can only occur when 

 T = T' ; that is to say, when the resistances of the distant and of 

 the balance thermometers are equal, or in other words, when their 

 temperatures are alike. 



In making an observation with this apparatus, it is therefore 

 only necessary to heat or cool the water in which T is immersed, 

 and to read off its temperature upon an ordinary mercury ther- 

 mometer the moment that electrical equilibrium is observed.. The 

 temperature thus noted is that of the distant station. 



THEIIMOMKTRIC COMPARISON-COIL. The comparison-coil, the 

 temperature of which has to be adjusted, consists of a coil of fine silk- 

 covered iron or copper wire, corresponding with the wire employed 

 for, and of a resistance precisely equal to, that of the thermometer- 

 coil at a standard temperature. It is wound upon a short length 

 of metal tube and enclosed in an outer protecting capsule of silver, 

 or other metal, to guard it against mechanical injury and against 

 the ingress of water, which, by causing short circuits between the 

 convolutions, would render its indications inexact. The open end 

 of the protecting capsule is fitted with a vulcanite stopper through 

 which two thick copper leading wires, forming the end of the 

 a i ice coil, are passed. 



The water bath used with this instrument, and which I have 

 found very convenient for raising or lowering the temperature of 

 the comparison-coil to that of the distant spot, consists of a cylin- 



VOL. II. M 



