,; ,.;. ||. <;i;imTHS AND MR. EZKR GRIFFITHS ON THE 



The value of R so determined requires a small correction, as it includes the entire 

 resistance of the two straight leads, previously mentioned, connecting the upper ends 

 of the coil with the potential junctions. 



Of the heat generated in these leads, a portion is lost by radiation, &c., owing to 

 their projecting 24 mm. above the surface of the oil. 



The resistance of the leads could not be diminished beyond a certain limit, on 

 account of thermal conduction along them of the heat from the hot oil. A diameter 

 of 1 mm. was decided upon, as the thermal conductivity of a manganin wire of this 

 size would be negligibly small. 



A certain amount of heat was developed in these short leads by the current. 

 That generated in the 20 mm. below the surface of the oil would undoubtedly be 

 absorbed by the oil. Of the heat generated in the 24 mm. above the oil-surface, it is 

 probab'e that about one-half would pass into the block, &c., by conduction and by 

 radiation to the ferrules and quartz tubes. 



Taking the actual figures : Resistance per millimetre of the wire = '000642. 

 Hence, resistance of portion above the oil surface = 0'0154 ohm. 



On the above assumption only the heat generated in half of this was, in the case 

 of either lead, effective in heating the block. We confess that this is merely an 

 assumption, but, with our knowledge of the actual conditions, it appears to be a 

 reasonable one ; moreover an error of 10 per cent, therein would only affect the 

 absolute value of our results by less than 1 part in 10,000. 



Change of Resistance due to Change in the Current. 



If R' is the resistance of the heating coil when a certain current (defined later) is 

 passing through it and R is the value determined in the usual manner by the dial 

 resistance box, then we define SE, by the relation 



The effect of the temperature rise (produced by the heating current) on the 

 resistance of the wire was of course very small in the case of an alloy like manganin ; 

 the resulting correction, however, could be determined with considerable accuracy in 

 the following manner (see fig. 5). 



A series of observations was made in which the current was measured by the 

 ordinary potentiometer method. Included in the circuit was a 3-ohm coil ( W) of bare 

 manganin wire immersed in stirred paraffin oil. It consisted of 4 strands of 0'4 mm. 

 diameter, in parallel, wound on eight projecting mica plates fixed longitudinally on a 

 wooden drum. The passage of the maximum current (0'45 ampere) for intervals of 

 several minutes did not produce any appreciable change in the temperature of the oil. 



One observer adjusted the current in the circuit until the potential difference at 



