42 MR. DANIELL ON VOLTAIC COMBINATIONS. 



electrolyte which I employed in all the following experiments was the same as that 

 which I employ in the constant battery, viz. eight parts of water, by measure, to one 

 part of oil of vitriol in contact with the generating surface, and the same diluted acid, 

 further saturated with sulphate of copper, in contact with the conducting surface. 



The measures of the force to which I had recourse were, in the first instance, a 

 coarse galvanometer consisting of a single needle seven inches in length, with a coil 

 of twenty turns of wireVo^th of an inch in diameter. The large scale of the experiments 

 precluded me from employing delicate instruments with astatic needles. Being dis- 

 satisfied with this measure, I afterwards employed a Breguet's thermometer fitted up 

 according to the plan of Professor De la Rive*, which measures the force of a current 

 which passes through it by the differences of its heating power upon the compound 

 spiral of platinum and silver of which it consists, the degrees into which it is divided 

 being directly proportional to such differences. I had every reason to be satisfied 

 with this instrument; and have no doubt of its accuracy, in the case, at least, of cur- 

 rents of such low tension as those for which alone I have hitherto employed it. 



In my first experiment the sphere was charged with the solution of copper, and the 

 membrane with the plain acid ; a small sphere of amalgamated zinc, one inch dia- 

 meter (and exposing a surface therefore of 3-14 square inches), was suspended by 

 means of a well-varnished copper wire in the centre of the latter : the other extre- 

 mity of the wire was connected with one of the cups of the galvanometer, and the 

 circuit was completed by a wire leading from the other cup to a small mercury cup 

 upon the upper brass hemisphere, placed at a distance of two inches from the tube. 

 The deviation of the needle was 60° ; and it remained steady for a long period, during 

 which the experiment was repeated and varied. When, instead of the galvanometer, 

 the circuit was closed with a piece of platinum wire one inch in length and -^-^ 

 inch diameter, it continued red hot for a period of five hours. The circuit remained 

 closed for seventeen hours, and the apparatus was then opened and examined. The 

 zinc ball had dropped off the wire, and was reduced to about one half of its original 

 size. Tlie upper hemisphere was found coated with reduced copper, beautifully marked 

 half-way up from the equator with concentric circles of alternate dark and light stripes 

 of pink and red ; these were followed by a broad even band of pink, which reached 

 to a circle within 1 J inch of the aperture, which was composed of the unchanged 

 surface of the brass, and which evidently had not been in contact with the liquid. 

 The lower hemisphere, which had been insulated from the upper by the collar of 

 leather, had no copper precipitated upon it. 



The sphere was again put together, and charged as before, with a new zinc ball. 

 The circuit was closed, as in the first experiment, with the galvanometer in contact 

 with the upper hemisphere, and the deviation of the needle was 60°. 



The connexion was then broken with the upper hemisphere, and made with the 

 bottom of the lower hemisphere ; the deviation was again 60°. 



* Memoires de la Soc. de Phys. de G^nfeve, 1836, p. 140. 



