216 MESSRS. W. R. BOUSFIELD AND W. ERIC BOUSFIELD 



mercury leads to the water is included in the heating effects, the determination of 



which is described in Section 7. 



The total resistance of the two mercury leads from the level of the baffle plate 

 upwards in the earlier range of experiments was 0'0045, and this was subtracted from 

 the total resistance of the thermometer-resistance M 3 in order to obtain the net 

 resistance of the heater for calculating the electrical energy. 



6. Considerations in Reference to Working Standards of Resistance for Carrying 



Heavy Currents. We must now turn aside for a moment in order to explain the 



reasons which led us to abandon the use of manganin or other alloys, and to design a 

 type of calorimeter in which the mercury thermometer-resistances could be con- 

 veniently employed. For quantitative measurements of electrical heating, unless one 

 could rely on absolute measurements of current used, it is necessary to work from a 

 standard resistance of known magnitude which carries the heavy current used for 

 heating. At an early stage of these experiments a standard 10-ohm manganin 

 resistance was procured which was made by the Cambridge Scientific Instrument 

 Company and designed to carry 5 amperes. It was intended to use this resistance in 

 conjunction with standard cells to determine the current. This resistance consisted 

 of two sets of coils of manganin wire in parallel. The diameter of the wire was 

 1'2 mm. and the length per ohm of the two wires was 1065 cm. This gave a total 

 cooling surface per ohm of about 400 cm. 2 , so that with a current of 5 amperes 

 through the resistance, developing 25 watts per ohm, the cooling surface was 16 cm.'' 

 per watt. The wire was wound on brass tubes of about 3 cm. diameter, and the 

 whole resistance was immersed in a large bath of paraffin oil provided with an 

 efficient stirrer. 



The standard resistance used by BARNES* consisted of eight wires of platinum- 

 silver in parallel of 0'4 mm. in diameter, and each 1 m. in length, immersed in oil. 

 Four watts were developed in each wire, so that the cooling surface was 3 '2 cm. 2 per 

 watt. He observes that " it was impossible to imagine that the temperature of the 

 wire could have been sufficiently different to that of the oil to appreciably affect 

 the resistance." BARNES' wires were bare. Ours had a thin coating of insulating 

 material, but a cooling surface per watt five times greater than his. We were at 

 first under the belief that the possible difference of temperature between the wire and 

 the bath might be a serious source of error in inferring the resistance under a heavy 

 working current from the temperature of the bath. This suspicion originally led to 

 the device of the mercury thermometer-resistance, the calibration of which has been 

 just described. When this had been installed, it became possible to use it in order to 

 test the value of the manganin resistance during the passage of a heavy current so as 

 to put our suspicions to the proof. We found, not the temperature difference which 

 we had suspected, but a result of a different kind. 



In fig. 5 the curve A is the temperature-resistance curve of the manganin standard 



* ' Phil. Trans.,' A, vol. 199, p. 180, 1902. 



