262 Duane — Heat Generated by Radio-active Substances. 



and quickly. Thus the thermal equilibrium of the apparatus 

 is not disturbed much by the heat generated by the source. 



This method is capable of considerable precision and can 

 be used, without changing the apparatus, to measure heat 

 effects varying from - 001 gram-calorie to 2 gram-calories per 

 hour. Larger heat effects could be measured by increasing 

 the thickness of the iron-nickel tube and iron and nickel wires 

 so as to decrease their electrical resistance and the heat gen- 

 erated in them according to Joule's law. 



The iron-nickel tube has been carefully standardized by 

 inserting a small coil of manganine wire of known resistance 

 into the tube, by heating this with a known electric current, 

 and by determining the current in the tube that would exactly 

 absorb the heat produced. 



The electric currents were produced by small storage 

 batteries, and their intensities were varied by changing the 

 resistances in plug resistance boxes contained in the circuits. 

 The resistances in these boxes, as well as the other resistances 

 in the circuits, were carefully measured by a standard Wheat 

 stone's bridge. The electric currents were measured by com- 

 paring the electromotive forces of the storage cells with that 

 of a standard Weston cell by the potentiometer method, and by 

 dividing these electromotive forces by the total resistance in 

 the circuits. 



The following table contains the data obtained in standard- 

 izing the iron-nickel tube. The resistance of the small coil 

 inserted into the calorimeter was 9'20 ohms, and that of the 

 lead wires attached to it was negligible. The electromotive 

 force of the standard Weston cell was 1,018 volts, and that 

 of the two cells forming the storage battery 4,153 volts. 



Table 1. 

 Total resistance Heat produced Current in iron- Heat absorbed 



in heating cir- in heating cir- nickel tube. per hour per 



cuit. cuit calories . Ampere ampere 



Ohms per hour , * 



Observed Corrected 



480 -593 -0716 -0716 8 29 



550 -450 -0540 '0542 8-30 



650 '324 -0392 '0393 8"25 



910 -165 -0200 -0197 8'37 



The compensations were not always exact, and a small cor- 

 rection was made in the values of the current in the iron- 

 nickel tube. This correction was determined by observing 

 the velocity of the bubble of air. 



The heat effect in the iron-nickel tube is due to two causes. 

 Firstly, the heat generated or absorbed at the junctions of the 

 metals according to the direction of the current (Peltier effect), 

 and, secondly, the heat generated according to Joule's law, 



