496 Profs. Aj-rton and Perry on the 



axis. The explanation of this seems almost certainly to be 

 an increase in the resistance of the liquid column, owing to 

 the presence in considerable quantity of the gaseous products 



in the path of the current. A straight line dotted thus 



has been ruled through those points which lie on the straight 

 part of the curve, to cut the axis of potential. Representing 

 this straight line by the equation E = <? + CR, and putting in 

 the numerical values of E and C, a series of equations was 

 obtained containing e and R. The mean value of e at 40° is 

 2*32, and at 80° 2"08 ; and these values of the intercept on the 

 axis of potential represent in volts the values of the maximum 

 steady electromotive force of polarization at these tempera- 

 tures under the conditions of the experiment. The difference 

 •24 volt, produced by a rise of 40° in temperature, gives '26 

 per cent, as the rate of variation per degree at temperature 

 40°. The other constant R of the straight line, representing 

 the tangent of its angle of inclination to the axis of current, 

 being the quotient of potential by current, indicates the resist- 

 ance of the voltameter under the conditions of the experiment. 

 The mean values of R obtained from the equations were for 

 40°, '69, and for 80°, '67 ; these numbers representing in ohms 

 the working resistance of the voltameter. 



Conclusions. — (1) The opposing electromotive force of 

 polarization which arises in cells when at work depends 

 on the value of the current passing through them when that 

 current is below a certain value, increasing, but more and 

 more slowly, with the current. 



(2) There is a maximum value of the polarization regarded 

 only as a function of the current-strength, beyond which any 

 increase in the strength of the current has no effect upon it. 



(3) The electromotive force of polarization varies with 

 temperature, its value decreasing about 1 per cent, for a rise 

 of temperature of 4° C. 



The method of experiment was suggested by Professor 

 Carey-Foster, in whose laboratory the experiments were 

 made. 



LXI. The Magnetic Circuit of Dynamo Machines. 



By W. E. Ayrton and John Perry.* 



[Plate IV.] 



IN this paper we shall use the following symbols : — 

 Dimensions are given in centimetres, current in amperes, 

 potential differences and electromotive forces in volts, re- 

 sistances in ohms 



* Communicated by the Physical Society : read March 10, 1888. 



