208 Mr W. S. HARRIS on the Heat excited in 



3. The indications of this instrument appearing, after a few 

 trials, to be very general and decisive, I endeavoured to ascertain 

 how far it might be relied on as a measure of electrical action, as 

 also the variations to which it might be subject : as these points 

 became satisfactorily determined in the course of the following 

 investigation, I shall give a brief detail of the experiments as 

 they occurred. 



4. (a) My first attempts were directed to the law of the ac- 

 tion of a single zinc and copper plate xy, Fig. 1, placed at various 

 distances from each other in a dilute acid. The plates employed 

 for this purpose were each 7 inches high, 6 inches wide, and 

 about the y- th of an inch thick, and were secured in a light 

 frame of varnished wood/"/! The cell A for the acid had bottom 

 and ends of varnished mahogany, and sides of glass ; it was in- 

 ternally 10 inches long, 8 inches deep, and 7 inches wide. The 

 wire in the electrometer B was of copper, and about the yio^h 

 of an inch in diameter *. The dilute acid consisted of nitrous 

 acid one part, sulphuric acid one part, water forty-eight parts by 

 measure. The contacts with the plates were effected through 

 small cups xy containing mercury ; the wires employed to com- 

 plete the circuit being of copper, and ^th of an inch in diameter. 



The following are the results of this experiment : 



be added as much sulphuric acid as will render the whole pleasantly sour. Mercury 

 also, when the tube is fine, may be used with advantage. 



* This experiment was likewise contemplated by my friend Mr WATKINS, the 

 curator of the apparatus, in the London University. In the course of a few trials 

 with the instrument which I had the pleasure of making with him at his house, we 

 succeeded in rendering sensible, the action of a small plate of zinc and copper, of 

 only ^th of an inch square, when placed in an extremely diluted acid, at upwards 

 of 4 inches apart ; the wire in the electrometer was in this instance extremely fine. 



