Laws of the Elementary Voltaic Battery. 41 



needle) is passed through another piece of straw, of rather greater 

 diameter, to allow the former to fit into it. By this construction, the 

 needle can readily be detached from the filament when the instru- 

 ment is not in use. The same straw which carries the needle is also 

 perforated by an index at right angles to it, consisting of a fine rigid 

 straw about five inches long, and having on its shorter end a little 

 counterpoise. This index moves over a short graduated arch in the 

 circle B at a very small distance above it. It is intended, when us- 

 ing the instrument, to bring this index to zero, by means of the tor- 

 sion key above. There being no obstruction to the view of the index 

 in this arrangement, the adjustment can be more accurately and 

 speedily made. A pin projects from the center of the circle B, and 

 enters a litde way the straw, to steady the motion of the needle. A 

 rim of pasteboard two inches high rises above the circle B, and the 

 whole, both needle and coil, are covered by two semicircular plates 

 of glass. The torsion key moving over the upper circle, which is 

 minutely graduated, denotes the amount of deflection when the index 

 below is brought to zero. 



I. On the relative influences severally possessed by the zinc and 

 copper in the elementary battery. 



Our first experiments on this point were made with a pair of short 

 plates graduated to the ^th of an inch. The quantities of surface 

 immersed are expressed at the top of each column of observations. 

 The numbers in the columns Indicate the angular deflexion of the 

 needle observed at intervals of one minute ; the first observation be- 

 ing made a minute after the plunge. After completing the first col- 

 umn of observations, the slips were removed for a new adjustment ; 

 and so on for the succeeding columns. No particular attention was 

 as yet paid to the length of interval during which the plates were out 

 of the action of the solution. The solution in nearly all our experi- 

 ments contained one part of sulphuric acid in about 100 of water. 

 1. Copper 2 J in. zinc dipped to 



* In this and some of the following experiments, the plates were transferred after 

 the completion of the series to a vessel containing fresh liquid of the proper strength. 



Vol. XXVII.— No. 1. 6 



