Dr. Andrews on the Properties of Voltaic Circles. 



153 



to be influenced by those changes in the platina surfaces. It must, however, be 

 particularly observed, that there is invariably less zinc dissolved with the larger 

 platina plate, — a result altogether at variance with the established laws of voltaic 

 action. 



To ascertain whether this apparent anomaly depended upon some peculiarity 

 in the mode of generation of these currents, or on the ordinary action of the 

 acid on the zinc being more completely checked by the broader platina surface, 

 it appeared to be necessary to determine the quantity of electricity actually 

 developed under these conditions. For this purpose a galvanometer, composed 

 of a pair of astatic needles, with a single silver wire between them, was inter- 

 posed in the course of the circuit. As the needle of such an instrument can 

 scarcely be maintained in a stationary position, but oscillates through an arc of 

 two or three degrees round a fixed point, the most accurate method of ascertain- 

 ing its deflection is to make five or more observations of the extremities of the 

 arcs through which it vibrates, and to take a mean of the whole ; and this was 

 the method followed in obtaining the deflections contained in the next table. As 

 each degree of the scale occupied only one-fortieth of an inch, and was not sub- 

 divided into smaller parts, it was difficult to avoid an error of a quarter of a 

 degree in making the observations. The second column of the annexed table 

 gives the extent of the surface of the platina exposed to the liquid, that of the 

 zinc being represented by 1 ; and the third column, the deflection of the needle 

 of the galvanometer. The temperature was 156° c. during the whole course of 

 the experiment, and the distance between the zinc and platina surfaces was one 

 inch and a half. 



The trifling differences in the deflections of the needle, in the first four expe- 

 riments, certainly depended upon the unavoidable errors of observation and 

 minute variations in the temperature of the acid. The current was therefore 



y2 



