U4 ,.. E. n. (;RIFFITHS AND MR EZER GRIFFITHS ox THE 



,' 4 I't), .-.IK! after slight adjustment* of the galvanometer spot to its zero mark, the 

 r 1 . l lv.. l M..iii.-iT k.-y was tunied so as to re-establish the bridge current (0'013 ampere). 



Tlie rise of temperature, as indicated by the movement of the spot, was uniform, 

 :md its transit was recorded by a tap on the chronograph key. The cycle of 

 operations was repeated; the transits of the temperature across each bridge-wire 

 nM<liiig being recorded in succession until the temperature had risen to +8 bridge- 

 wire reading, when the current was switched over to the auxiliary coil. 



After the current had ceased to impart any heat to the metal, the observed 

 temperature continued to rise, on account of excess of heat in the oil, the gradient 

 from the interior to the surface of the metal and the temperature lag of the thermo- 

 meter. 



The metal would, however, after its temperature had risen to a maximum, part 

 with its heat by radiation, &c., only, the resulting fall being slow and regular. This 

 " rise above," as we termed it, could be accurately determined by the following 

 procedure: 



The bridge contact-maker was set above the switching-off point by an amount such 

 that the galvanometer spot moved to near the centre of the scale before the regular 

 cooling l>egan. The galvanometer deflections on reversal of the bridge current were 

 noted, and also (on the chronograph tape) the times of the observations, until the 

 deflections had increased beyond the range of the scale. The value of 1 mm. scale 

 deflection in terms of a bridge-wire unit being known, f the rate of fall in temperature 

 could l)e determined, and the temperature time-curve P... ABCD could be constructed. 



One of the resulting diagrams for the " rise above" is shown in fig. 6. 



If P is the point at which the current was switched off, the slope of the line CD, i.e., 

 the rate of uniform cooling, gives the data required for the determination of the 

 horizontal line EG, and thus the temperature which the metal would have attained, in 

 the alwence of radiation, &c. , can be ascertained. 



If GE l>e produced liackwards to meet the temperature ordinate at F, then it will 

 be evident that F falls on DC produced. 



Thus PF, the rise due to the residual heat in the block, could be determined at the 

 close of each experiment with considerable accuracy. The value rarely exceeded 

 0'l Pt and could be measured to 1 part in 1000, that is, about 1 in 15,000 of the 

 whole range. 



It may be mentioned that the " rises above " for a series of experiments with the 

 same metal under the same conditions were proportional to n a , i.e., to the rate of supply. 



The galvanometer system generally required this slight readjustment between each observation of 

 transit in order to maintain the spot on the scale zero when the bridge current was broken. This was 

 effected by the movement of a small subsidiary control magnet on the table by the observer, and about 

 1 J m. distant from the galvanometer. The changes of zero were chiefly those due to variations in the 

 thermoelectric effects in the circuit, and with considerable attention to shielding the various junctions w 

 succeeded in diminishing such changes to small dimensions, but could not altogether eliminate them. 

 t Ascertained for each get of experiments. 



we 



