490 harper: specific heat of copper 



was that which was comprehended in the thermometric factor, 

 and the energy suppHed to it during the period of heating was 

 the product of the current in the coil by the potential drop 

 between these leads, integrated over the time the heating circuit 

 was closed. This potential drop was measured directly with a 

 potentiometer and the current was measured with the same 

 potentiometer balanced on the drop across a 0.1 ohm resistance 

 standard connected in the series in the circuit. The time factor 

 was obtained by automatic chronograph record of the instants 

 when the heating current was switched on and off. 



The specimen was heated 4° to 5° in each experiment, the rise 

 in temperature being determined by the change of resistance of 

 the specimen relative to the resistance of a 0.1 ohm manganin 

 resistance standard immersed in an oil bath. The comparison 

 was effected by a potentiometer, sensitive to about one part in 

 five hundred thousand, and the current employed in making the 

 comparison was small enough (about 0.3 amp.) that its heating 

 effect on the specimen (about 0.001 per minute) could readily 

 be allowed for without appreciable error. 



To obtain the factor for the reduction of the increase in re- 

 sistance to rise of temperature on the international hydrogen 

 scale, the copper thermometer was compared at several tempera- 

 tures with two standard platinum thermometers in a stirred 

 oil-bath comparator. 



By suspending the specimen in vacuo, the uncertainties in the 

 cooling correction which occur at atmospheric pressure due to 

 convection were avoided, the magnitude of the correction was 

 increased, and uncertainty in the amount of air to be included 

 in the water-equivalent (a source of error barely appreciable) 

 was avoided. The efficacy of the vacuum jacket in reducing the 

 magnitude of the cooling correction was by no means what was 

 anticipated. 



The results of 27 determinations at temperatures between 15° 

 and 50° possess an average deviation of one part in a thousand 

 millimeters from 0.3834 -|- 0.00020 (t-25) international joules per 

 gram degree; equivalent to 0.0917 + 0.000048 (t-25) calories2o 

 per gram degree, if 4.182 joules equal one (20°) calorie. 



