the same thing), or (b) alternating. The E.M.F. or tension corresponds to pressure, to use the mechanical analogy of a water pipe, and the ampèrage to volume, and the voltage of the supply currents in this country are usually between 100 and 250 volts. Private lighting sets are frequently as low as 50, and current derived from accumulators may be anything from a few volts and upwards. Power currents, such as commonly employed for tramways, &c., are usually about 500 volts, but the use of these currents for lighting purposes, though practicable, is not to be advocated.
Ampères and volts are convertible terms in a sense; that is to say, a current of 10 ampères at 100 volts requires the same horse-power to generate it as one of 5 ampères at 200 volts, or 20 ampères at 50 volts, but they are by no means convertible as regards their efficient use for our purpose. The ampères used multiplied by the number of volts give the total power consumed in watts, and 1000 watts used for one hour represent 1 unit as charged for on our dreaded lighting bills. The current available from a public supply may be said to be unlimited so far as our purpose is concerned, and the amount actually used depends only on the total electrical resistance of our circuit, and this is measured in ohms, the three factors, viz. volts, ampères, and resistance, being connected by the well-known and simple equation C = E / R, C representing the current in ampères, E the tension or E.M.F. (electro-motive force) in volts, and R the resistance in ohms. The total current we can use, however, is limited by the size of the cable laid on in the building, and this is automatically safeguarded (or should be) by the fuses, which consist, as is generally known, of thin wires or strips of tin or lead fixed on a fuse board in an easily accessible place, and which melt directly the current exceeds a safe amount in ampères. Whatever method of lighting we use therefore, enough resistance must always be kept in the circuit to ensure
that no more current can pass than has been provided for, and in the case of an arc lamp this usually means a resistance or rheostat being retained in the circuit in addition to the arc itself, through which the current is passed and absolutely wasted, though fortunately the waste in money is negligible, and for reasons to be discussed later such a resistance is necessary with an optical lantern arc lamp in any case.
In the case of a glow-lamp, the entire resistance is provided by the filament of the lamp itself, and that is why an ordinary metal or carbon filament lamp, for say 200 volts, has to be manufactured with an extremely long and slender, and therefore fragile, filament, while with an ordinary pocket-torch, which is usually supplied with current from a dry battery of some 3 or 4 volts only, the filament can be short and thick.
Speaking generally, glow-lamps on a low voltage current can be made more efficient than on a high one, and are also longer lived for very obvious reasons; but, on the other hand, the transmission of current over long distances is cheaper the higher the tension, as for a given number of watts the ampèrage is less, and therefore smaller cables can be employed. On the whole, then, currents of 200 to 250 volts have during recent years become more common than 100, in spite of greater difficulties in making the lamps; but occasionally one finds a hall where two or more lamps are wired in series, two 100-volt lamps for example being wired together in series on a 200-volt circuit. If we are using current for our lantern from an ordinary lamp socket, this is a possibility that must be borne in mind.
The same considerations, viz. the economy of transmitting power at high tension and of using it at a lower one, have been mainly responsible for the rapidly increasing number of alternating current circuits now met with, especially in sparsely populated districts. An alternating current main is one in which the current reverses its direction, usually in this country 50, but sometimes 60, 80, 90, or even 100 times
per second (there being unfortunately in Great Britain no standard 'Periodicity' or number of cycles per second), and for technical reasons which need not be entered upon here, with these alternating currents the tension and ampèrage can be mutually converted by means of transformers, so that current can be transmitted at so high a tension, for instance, as 10,000 volts, and used at a voltage of 50 or 100 or whatever is required, the ampèrage available being increased in inverse ratio as the tension is decreased. The same ready power of transformation unfortunately does not apply to the continuous current, or alternating currents would probably never have been heard of, but as it is they are very common. For glow-lamps it is immaterial which current is available, but for arc lamps the continuous is much to be preferred, though both can be utilised.
With these initial remarks, I will now take in order of illumination the various methods of utilising the electric current for optical lantern work.
The Electric Glow-Lamp.—The ordinary metal filament lamp is not very suitable for lantern work, the light not being sufficiently concentrated, but from what has already been said, it will be evident that this method of lighting is more suitable where currents of low voltage are available.
An extremely good and intense light can be obtained from a comparatively small battery of accumulators, which can easily be carried in the hand, and a short and thick metal filament lamp, similar to those supplied with a powerful electric torch; and this arrangement is actually used to some extent by travelling lecturers, but the mess and trouble of keeping the accumulators in order have prevented the method being generally adopted.