348 



Messrs. McLennan and Burton on the 



quadraiUs would attain a steady potential, the current between 

 the cylinders being equal to that between the connecting wire 

 and its surrounding tube. The potential assumed by the free 

 quadrants under these circumstances, while approximating to, 

 would be less th^n the contact difference of potential for the 

 metals used in the two cylinders. 



The values obtained in this way for any two given metals, 

 while differing in sign, should be numerically the same 

 whichever metal was used for the inner or outer cylinder. 

 But on trial ^vith a number of pairs of metals it was in- 

 variably found that the readings differed when an interchange 

 was made of the metals in the cylinders. This is illustrated 

 bv the curves in fior 3. which o:ive the results for the metals 



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zinc and copper, the upper representing the negative potentials 

 acquired by the quadrants with an inner cylinder of zinc, and 

 the lower the positive potentials with one of copper. The 

 final potentials, it will be seen, are 527 and 304 millivolts 

 respectively. 



This result finds its explanation in the metal effect described 

 above. We have shown that with zinc cylinders the inner, 

 in virtue of this effect, attains a potential of 160 millivolts 

 below that of the outer, while with copper the corresponding 

 value is 73 millivolts. If, then, we suppose the two '■ effects " 

 to act concurrently, the final readings observed with the zinc 

 and copper cylinders will represent their algebraic sum. 



