In the course of the popular articles on frictional and voltaic electricity, it has already been mentioned that whilst the _intensity effects_--such as the capability of the spark to pass through a certain thickness of air, or the production of the peculiar physiological effect of the shock--belong especially to the phenomena of frictional electricity, they are not apparent with the _quantity effects_, such as may be produced by an ordinary voltaic battery, unless the latter consists of an immense number of elements, such as the famous water battery of the late respected Mr. Crosse, which consisted of two thousand five hundred pairs of copper and zinc cylinders, well insulated on glass stands, and protected from dust and light. If, however, the feeble intensity current of voltaic electricity, from four or five elements, is permitted to pass into a coil of a peculiar construction, fitted with a condenser, and manufactured either by Ruhmkorff of Paris, or Mr. Hearder of Plymouth, then the most remarkable effects are producible, which have created quite a new and distinct series of phenomena, and further established in the most satisfactory manner the connexion between the electricities derived from _friction_ and _chemical action_. The construction of these coils does not differ very materially, and great merit is due to Messrs. Ruhmkorff, Hearder, and Bentley, who have separately and independently worked out the construction of the most formidable machines of this class. In a letter to the author Mr. Bentley says:-- "I commence the formation of my coil by using as an axis an iron tube ten inches long and half an inch diameter; around this is placed a considerable number of insulated iron wires the same length as the tube, and sufficiently numerous to form a bundle one inch and three quarters diameter. This core is wrapped carefully in eight or nine layers of waxed silk, the necessity of which will be obvious presently. "My primary helix, which is formed of thirty yards of No. 14 cotton-covered copper wire, is wound carefully on this core, and consists of two layers, each layer being carefully insulated one from the other by waxed silk, for I find that if a wet string or fine platinum wire be connected with the two ends of the primary wires of an induction coil in action, there is scarcely an indication of an induced current to be obtained from the secondary wire. That this is not owing to any decrease of magnetic power is proved by testing the iron core before and after the experiment, but is simply owing to the central magnet or coil exerting the whole of its inductive powers upon the nearest closed circuit; it therefore follows that if the two layers of primary wire are connected by the cotton covering becoming moist, the whole of the [Page 231] induced current will take this path instead of traversing the secondary wire.