.V/A 1 WILLIAM SIEMENS, F.R,S. 



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lin'.Tt-ntial voltameter. The method of measuring electrical resist- 

 ances by the aid of this instrument will IKJ described in the Third 

 Part of this Paper. 



Although the principle involved in the increase of electrical 

 resistances with increasing temperatures is an extremely simple 

 on-', the difficulties which had to be overcome in constructing 

 1 tract it-ally useful instruments for measuring high and low tem- 

 pi-rat ures, were considerable. Various combinations and appliances 

 had to be tried for protecting the thermometer coils agaiust hydro- 

 static pressure, or against the destructive heat of furnaces. The 

 disturbing effect of leading wires had to be eliminated, and the 

 reading of the instrument rendered independent of mechanical or 

 magnetic influences, and brought within the compass of observers 

 untrained for the delicate work of the electrician. 



But the greatest drawback consisted in the imperfect state of 

 electrical science respecting the ratio of increase of electrical re- 

 sistance with increase of temperature, for temperatures exceeding 

 the boiling point of water. Platinum is the only available metal 

 for high temperatures, and little was known of the ratio of increase 

 of this metal even at ordinary temperatures. I was, therefore, 

 obliged to undertake the series of experiments, with the view of 

 determining the increase of platinum resistance up to high tem- 

 peratures, tending to the establishment of the general law with 

 regard to electrical resistances which has been dealt with in the 

 First Part of this Paper. 



The resistance thermometer and pyrometer have already been 

 applied to useful work. Professor Bolzani, of Kasan, uses them 

 for registering cosmical temperatures at points above and below 

 the surface of the earth. Mr. I. Lowthian Bell, the eminent 

 metallurgist, employs the latter for determining the temperatures 

 at which the various operations of the blast furnace are carried 

 on ; and I have had various occasions, in addition to the one 

 already referred to, of obtaining useful information regarding 

 the temperature of furnace gases, &c., by the aid of these 

 instruments. 



