502 



MR. J. E. PETAVEL ON THE HEAT DISSIPATED BY 



1. A thin wire maintained at any temperature above 1200 is subject to a rapid 



increase of resistance. 



2. It is impossible to obtain consistent results with a thin wire at atmospheric 

 pressure, owing to the constant fluctuation in temperature due to convection 



currents. 



3. As pointed out by Professors AYBTON and KILGOUR, with fine wires the 

 emissivity, referred to unit of surface, depends largely on the diameter, but the 

 rate of change decreases rapidly as the diameter increases. 



The diameter of the wire used was '112 centim., or nearly 1 sq. millim. in cross- 

 section. The resistance of the thermometers was about '01 ohm at C., or about 

 one five-hundredth of the resistance usually employed. 



Fig. 1. 



ST 



(Standard Resistance) 



To Che Potentiometer 



Jo the Potentiometer 



The electrical connections can be briefly described as follows : At C, in fig. 1, is 

 a set of forty cells, which by the aid of a mercury commutator can be placed in any 

 desired combination, either series or parallel. This commutator was used to make 

 the first rough adjustment of the current. R is a variable resistance consisting of 

 thirteen German silver tubes, which are kept at a constant temperature by a water 

 circulation. The resistance is designed to carry up to 150 amperes. By two sliding 

 contacts, one of which serves as a fine adjustment, the current can be regulated to 

 within '01 of its value. The current through the radiator is measured by aid of the 

 standard resistance ST, the exact value of which, according to the Keichsanstalt 

 certificate, is '0099965 ohm at 16'55 C., its temperature coefficient being '0003 per 

 cent. A small motor was used to maintain a circulation of oil round the wires of 

 this resistance, the oil being in its turn cooled by a water circulation. By this 

 means the temperature of the standard was prevented from rising above 25 C. 



