FAILURE OF OHM S LAW AT HIGH CURRENT DENSITIES. 



155 



Effect of Speed of Cooling IVatcr on Steady Temperature Rise. For 

 low speeds of cooling water with a constant D.C. the rise of tempera- 

 ture decreases rapidly with increase of speed. At higher rates of flow, 

 however, the effect becomes much less, until above a certain point 

 there is very little dependence of steady temperature rise on the rate 

 of flow. In the actual experiments the speed was chosen high enough 

 to be within this insensitive range. This ensures that conditions on 

 different days with different samples were rather closely the same. 

 There were slight outstanding dift'erences in the cooling conditions 



J0005 .001 .0015 .002 



Figure 5. Results obtained with gold 8X10"" cm. thick. The abscissae 

 are reciprocals of the frequency and the ordinates difference between A.C. 

 and D.C. settings in cm. of bridge wire. The two curves were obtained with 

 the same D.C. but with different heating effects, produced by varying the 

 rate of flow of the cooling water. The two curves should extrapolate to the 

 same point if there is a genuine departure from Ohm's law. 



of the different samples, however, due to slight-changes in the position 

 of the sample in the holder, changes in the angle of presentation of the 

 nozzle of the cooling stream, etc. 



Dependence of Stcadij Temperature Rise on Rate of Heat Input. Our 

 dimensional analysis, subject to the restrictions assumed for the 

 manner of flow of the cooling water, has" shown that the steady rise of 

 temperature does not depend on the metal, but only on its breadth, the 

 rate of heat input, and several other factors which do not change if 

 the cooling fluid is always water, and its velocity is in the range where 

 it does not much affect the cooling. Now the rate of heat input is 

 proportional to the square of the D.C. and may be computed in terms 

 of the resistance and the dimensions of the specimen. This computa- 

 tion was made, and the results plotted for a dozen specimens, com- 

 prising gold of the three thicknesses and silver. 



In the first place the steady temperature rise for all these specimens 



