The Optical Magic Lantern Journal (March 1897)

The Optical Magic Lantern Journal and Photographic Enlarger. 49 owing to the friction on the stream of water by the walls of the tube in which it is conveyed, and this friction or ‘‘resistance ”’ will be proportional to the roughness of these walls, or the material of which they are composed, and also to the length of tubing. It is the same with electricity. The amount which will pass around a given circuit is in direct proportion to the pressure which drives it—measured in volts—multiplied by its quantity or current—measured in amperes—and inversely proportional to the sum of the various resistances which it meets with on its journey. Electrical resistance is measured in ‘ ohms’”’ and the ohm is interchangeable, as it were, with the volt and the ampere. There are three factors, therefore, in every electrical circuit, through which electricity is passing, and if you know two of them you can find the third by the simplest deduction. Thus, if there be a total resistance of five ohms in a circuit in which the dynamo or other source of electricity is creating a pressure of 100 volts, you may be absolutely certain that the current is no more or less than 20 amperes. Here is a simple little formula—the invention of some American genius—which is easy : to remember, and which puts the whole thing into a nutshell. E. OR. K. stands for electro-motive force or voltage ; C. for the current in amperes, and R. for resistance, which is measured in ohms. To use this ingenious device, when you want to find one factor, the other two being known, you place your finger over the letter which represents the unknown quantity and multiply or divide the others with one another, as indicated by their position. Now let us see what all this means to the tanternist. In the first place it requires a certain minimum pressure of electricity before an arc can by any means be made to form between the carbon points. There is, as it were, a certain amount of resistance in the arc which must be overcome and subdued before you can do any; thing. After that, any increase in pressure will serve to drive a proportionately larger amount of current across the space between the carbons, and the quantity of light emitted depends upon the amount of current that passes. ‘T'wenty-five large Grove battery cells will yield a fine arc light, and one might suppose that twelve similar cells would give a light of nearly half the , brilliancy. But it is not so, for the potential of | twelve such cells falls far below the limiting number of volts, and no arc at all can be established. An arc light for lantern work, where the lamp employed is one of good and efficient make, so that the greatest possible amount of the total light which it yields finds its way through the system of lenses, a current of ten amperes will give a brilliant. illumination far surpassing that of even the best limelight jet. Now the resistance of the arc when running at about ten amperes— for in this one case the resistance varies with the strength of the current—is approximately three ohms. If such alight is to be run upon a 100 volt circuit, where the wire ‘‘ leads” are so sufficiently large, as they usually are, that their resistance need hardly be taken into consideration, an artificial resistance of 7 ohms will have to be included in the circuit in order to cut down the current to the required ten amperes. It will be seen that as the quantity of current that flows around a circuit is directly proportionate to the pressure divided by the resistance, if there be not sufficient of the latter, you will get a far larger current than you will know what to do with. For instance, if on a hundred volt circuit you only have a resistance of one-tenth of an ohm, directly you switch the current on you will have a rush of electricity of one thousand amperes, and that will require a copper wire one inch in diameter to carry it properly. If you attempt to send an electric current through a wire that is too small to convey it, that wire will get so hot that it will probably be melted before you have time to switch the current off again. It might easily happen, and indeed it often does, that owing to the wires accidently touching one another, the resistance of the circuit becomes in a | moment reduced far below its proper amount, and the consequence is an immense flow of current that—were it not for a certain safety device—would speedily melt up the wires, and probably set the place on fire. It is to obviate these disastrous results of an accidental ‘‘ short circuit” as it is called, that the supply companies always put a ‘“‘ fuze” on their leads when they bring them into your house, and they lock it up so that you cannot get at it. A ‘fuze ” is a very simple device. It consists merely of a short piece of tin wire, of a thickness proportional to the maximum current that it will be required to carry, which, owing to its easy fusability, immediately melts when more ' than the normal current passes through it, and dropping out from its terminals, automatically cuts off your supply of electricity. (To be continued.)