Decomposition of Water by the Galvanic Battery. 87 



changed more frequently than the lime ; and there is more reason 

 to fear that the coke light will fail ou account of the destruction 

 of the positive coke point, than that the lime light will go out 

 on account of the wearing of the lime. For the coke light, the 

 zinc plates should not contain more than 8 square inches of sur- 

 face. If they be larger, the positive coke point will be rapidly 

 destroyed, without a proportional increase of light. 



The sixth result is a new mode of exhibiting the dissolving 

 views by means of the lime light. In the common apparatus for 

 showing the dissolving views, the apertures of the lanterns are 

 gradually opened and gradually closed, in order to make the 

 figures come gradually into view and gradually disappear. By 

 using the mixed gases, the apertures may be left always open, 

 and the figure in either lantern may be gradually brought out 

 on the screen, and may be made to fade away by degrees ; for 

 by opening very slowly the stopcock through which the gases 

 pass to the jet, the light in the lanterns may be made to increase 

 very slowly till it has acquii-ed its full intensity, and consequently 

 the image thrown on the screen will become gradually brighter 

 till it attains its full brightness ; and by very slowly closing the 

 stopcock, the light, and consequently the image, will gradually 

 fade away. By exhibiting the dissolving views in this way, there 

 is a great saving of the mixed gases ; for the light shines out 

 fully in each lantern only when it is necessary to exhibit the 

 image in full brightness on the screen. Besides, by diminishing 

 the quantity of the gases which passes through one jet, the quan- 

 tity which passes through the other jet, and consequently the 

 brightness of the full image, is increased. 



The last result is a new sine galvanometer, which is the only 

 instrument yet devised for measuring with accuracy very power- 

 ful galvanic currents*. Some of my experiments raised doubts 

 in my mind about some of the generally received theories relating 

 to the action of the galvanic battery. The galvanometers which 

 I had were not fit for measuring very powerful currents, such as 

 I employed. I was therefore obliged to get a new one. After 

 a good deal of reflection, I resolved to get one Nvhich would 

 answer for a sine instrument, for the common tangent instru- 

 ment, and for the new tangent galvanometer, the principle of 

 which is demonstrated in the Compies liendus of the 24th of last 

 January. Tlie galvanometer which I have made consists of a 

 mahogany circle 2 feet 1 inches in diameter and nearly 2 inches 

 thick, in the circumference of which is turned a groove half an 

 inch wide and 3^ inches deep ; of seven concentric coils of ^-inch 

 copper wire in the groove, and well insulated from each other; of 

 u strong frame in which the circle is moveable on an axis, and 



* The voltameter will not measure jwwerful currents |)ro(liicecl by a 

 Hinglu circle, or by two or three circles. 



