Electric Bells and All About Them: A Practical Book for Practical Men

by Selimo Romeo Bottone

§ 6. If the experiment described at [§ 3] be repeated, substituting a glass rod for the ebonite comb, it will be found that the pith-ball will be first attracted and then repelled, as in the case with the ebonite; and if of two similar pith-balls, each suspended by a fibre of silk, one be treated with the excited ebonite and the other with the glass rod, until repulsion occurs, and then approached to each other, the two balls will be found to attract each other. This proves that the electrical condition of the excited ebonite and of the excited glass must be different; for had it been the same, the two balls would have repelled one another. Farther, it will be found that the rubber with which the ebonite or the glass rod have been excited has also acquired electrical properties, attracting the pith-ball, previously repelled by the rod. From this we may gather that when one body acting on another, either mechanically or chemically, sets up an electrical condition in one of the two bodies, a similar electrical condition, but in the opposite sense, is produced in the other: in point of fact, that it is impossible to excite any one body without exciting a corresponding but opposite state in the other. (We may take, as a rough mechanical illustration of this, the effect which is produced on the pile of two pieces of plush or fur, on being drawn across one another in opposite directions. On examination we shall find that

both the piles have been laid down, the upper in the one direction, the lower in the other.) For a long time these two electrical states were held to depend upon two distinct electricities, which were called respectively vitreous and resinous, to indicate the nature of the bodies from which they were derived. Later on (when it was found that the theory of a single electricity could be made to account for all the phenomena, provided it was granted that some electrified bodies acquired more, while others acquired less than their natural share of electricity), the two states were known as positive and negative; and these names are still retained, although it is pretty generally conceded that electricity is not an entity in itself, but simply a mode of motion.

§ 7. It is usual, in treatises on electricity, to give a long list of the substances which acquire a positive or a negative condition when rubbed against one another. Such a table is of very little use, since the slightest modification in physical condition will influence very considerably the result. For example: if two similar sheets of glass be rubbed over one another, no change in electrical condition is produced; but if one be roughed while the other is left polished, this latter becomes positively, while the former becomes negatively, electrified. So, also, if one sheet of glass be warmed, while the other be left cold, the colder becomes positively, and the latter negatively, excited. As a general law, that body, the particles of which are more easily displaced, becomes negatively electrified.

§ 8. As, however, the electricity set up by friction

has not hitherto found any practical application in electric bell-ringing or signalling, we need not to go more deeply into this portion of the subject, but pass at once to the electricity elicited by the action of acids, or their salts, on metals.

Here, as might be expected from the law enunciated above, the metal more acted on by the acid becomes negatively electrified, while the one less acted on becomes positive.[2] The following table, copied from Ganot, gives an idea of the electrical condition which the commoner metals and graphite assume when two of them are immersed at the same time in dilute acid:—

The portion immersed in the acid fluid.	⎧	↓	Zinc.	↑	⎫	The portion out of the acid fluid.
	⎪	↓	Cadmium.	↑	⎪
	⎪	↓	Tin.	↑	⎪
	⎪	↓	Lead.	↑	⎪
	⎪	↓	Iron.	↑	⎪
	⎪	↓	Nickel.	↑	⎪
	⎪	↓	Bismuth.	↑	⎪
	⎪	↓	Antimony.	↑	⎪
	⎪	↓	Copper.	↑	⎪
	⎪	↓	Silver.	↑	⎪
	⎪	↓	Gold.	↑	⎪
	⎪	↓	Platinum.	↑	⎪
	⎩	↓	Graphite.	↑	⎭

The meaning of the above table is, that if we test the electrical condition of any two of its members when immersed in an acid fluid, we shall find that the ones at the head of the list are positive to those below them, but negative to those above them, if the test have reference to the condition of the parts within the fluid. On the

contrary, we shall find that any member of the list will be found to be negative to any one below it, or positive to any above it, if tested from the portion NOT immersed in the acid fluid.