electrotypers are unable to obtain or afford this. The cost of depositing copper by the dynamoelectric machine is very low, which soon repays the first cost of the machine in establishments where plenty of work can be constantly provided.

Thus a good machine, requiring the driving power of a small gas engine, or from two to three horsepower, may be made to work two or more vats of solution at once, the current being divided between them in the ratio of their resistances.

All the dynamo-electric machines mentioned in this volume (p. 87) yield a very strong current, and may be made to produce numbers of electrotypes with great rapidity. Some of the machines produce currents equal to those from thirty large Bunsen cells. In the frontispiece are represented Maxim's and Weston's machines, adapted for electro-typing. Figs. 9, 10, and 11, represent Gramme's large and small machines and Wilde's long-armature machine.

Provided that there is an abundance of work to do, there is no more economical source of the large currents required than a good dynamo-electric machine. The first cost varies from £40 to £100, and upwards. This is speedily recovered when the apparatus can be kept at full work. The cost of maintenance, apart from motive power, is a mere trifle, as nothing is consumed in the action.

The currents produced by these machines are derived from electro-magnetic force; they are, in short, the effect of the motive power expended, as current electricity. A full statement and explanation of their action will, however, be found in the author's treatise on "Electric Light" (Lockwood & Co.), to which the reader is referred for special information respecting them. Much care should be devoted to the selection of a dynamo-electric machine to suit the work to be done. Some general directions will be found respecting this in the present section of this volume.

Establishing a Dynamo-Electric Machine.-Ma

chines which require from three to ten horse-power to drive them are usually of a heavy description, and should be bolted either to wood beams secured to the earth or to masonry; the chief point is to secure a reliable fixture for such heavy machines, so that the vibration attendant upon high speeds

may be reduced to a minimum. Much care must, however, be taken to set the machine in a dry situation, because damp not only attacks its naked iron work, but frequently penetrates and injures the insulating covering of the wires. Smaller machines than the sizes above specified may advantageously be fixed securely to a raised bench

near to a wall, or to a stout wooden framework raised two or three feet from the floor level; it is

Fig. 11.-Wilde's Machine-Ena Elevation.

important, however, to avoid a weak base-work as much as possible, because a shaky machine frame

frequently sets the whole workshop into violent vibration, besides giving rise to much noise.

Most machines are simply provided with a fast pulley, so that in cases where the necessary counter-shaft is not sold with the machine, as in Gramme's apparatus, it is necessary to select a fast and loose pulley arrangement, of a size suited to give the required number of revolutions per minute to the machine. Broad and well-stretched bands should be employed, so that the tendency to slip may be reduced to a minimum. The number of revolutions given by the main shaft or engine should be calculated in relation to that run by the counter-shaft, and the relative sizes of both pulleys should then be considered to give the stated number of revolutions as an average, while the machine is at full work depositing copper.

Motors for Dynamo - Electric Machines. - The steam-engine is the best and cheapest motor for these machines, but both gas engines and watermotors are used. The chief peculiarity of the dynamo-electric machine is that to secure steady currents it must be driven steadily. This, however, in electro-typing may to a certain extent be overlooked. A gas engine which is not a very steady motor answers the purpose; those of Crossley Bros. are, probably, the best yet tried. Both gas and steam-engines should have a good margin of power over that absolutely required for driving the machine.

In many cases the machines may be situated