develops and finally becomes as strong as those of the sphere. Then the third conductor refuses to take any of the negative charge of the cylinder, and the process of accumulation ceases. If we wish it to go on further we must increase the source from which the negative charge of the cylinder is derived, that is, we must charge the sphere more strongly. How are we to increase this positive charge? The most obvious thing is to increase it also by mechanical means, much in the same way as we increase the charge on the third conductor, and this is the principle on which the modern electric machines, the so-called "influence machines," work.
The process is not carried out in the primitive manner here explained merely by way of elucidating a principle, but still the essential feature is retained of a go-between, or carrier of small charges, between the two conductors on which the electricities of opposite sign are to be accumulated. A familiar example of a practical way of making use of this principle is the electrophorus, but I shall not discuss it here, as it may be found in any elementary textbook. I prefer to deal at once with apparatus in which the principle of accumulating action is carried on
