292 DR. RITCHIE’S EXPERIMENTAL RESEARCHES 
19. The law now established may be exhibited to the eye by the co-ordinates 
of a parabola. Let V B (fig. 5.) represent the length of a battery, and the ordinate 
B C its voltaic power; and let V E be the length of another battery, the plates 
being equal and placed at equal distances, and let DE be its voltaic power 
then will F G represent the power of a battery whose length is V F. When 
the length of the battery becomes great, the length of the ordinates diminishes 
more rapidly than in the parabola, or the curve approaches more to the nature 
of an ellipse. For want of a battery of a sufficient number of plates, I have 
been unable to determine the real nature of the voltaic curve. It appears ex¬ 
ceedingly probable that the curve returns into itself, or, in other words, that the 
battery after gaining a certain power gradually loses its energy by any further 
increase of the number of plates. I should have scarcely any hesitation in 
touching the ends of a battery a mile long, and still less if it were extended to 
the length of ten miles. 
PART III. 
APPLICATION OF THE PRECEDING PRINCIPLES TO FURTHER DEVELOPMENTS 
IN VOLTAIC ELECTRICITY. 
20. There are only three substances which can be regarded as good con¬ 
ductors of voltaic electricity : viz. the metals, charcoal, and acidulated water. 
The metals when employed as conductors seem to have been the only sub¬ 
stances whose deflecting energy on the needle has been carefully examined. 
It appeared to me worth an experiment to ascertain if charcoal deflected the 
needle, and that, too, with the same energy as metal conducting an equal quan¬ 
tity of voltaic electricity. This was ascertained by the following arrangement. 
Exp. VIII. A piece of charcoal about an inch and a half long was placed be¬ 
tween two slips of copper fixed perpendicularly in a piece of wood, and an 
astatic needle, suspended by a fibre of silk, brought over the middle of the 
charcoal. Fig. 6. will exhibit this arrangement, in which AB is the charcoal, 
c, c' the slips of copper, w, w' wires soldered to the copper slips. When the 
wires were connected with an elementary or with a compound battery, the 
needle was deflected in the same manner as by a metallic wire. When one of 
the slips of copper was placed below the needle and at the same distance as 
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