lines of force in a continuous manner without any reversals in the direction of the induced currents. Such machines, sometimes called "uni-polar " machines, have, however, very low electromotive-force.

All and any of the continuous-current magneto-electric and dynamo-electric machines can be used as electromotors, the armature rotating with considerable power when a current from an independent source is led into the machine.

411. (b) Edison's Machine.-Some very large dynamoelectric generators have been constructed by Edison for his system of electric lighting. This machine (as shown in Fig. 154) is built upon the same bed-plate as the steam-engine (of 120 H-P) which drives it, and is called by its designer the steam-dynamo. The field-magnets are placed horizontally, and consist of 12 cylindrical iron bars overwound with wire, united to solid iron pole-pieces weighing many tons. Between the upper and lower pole-pieces rotates the armature, which is a modification of the drum-armature of Siemens, and is made up of 98 long rods of copper connected by copper discs at the ends instead of coils of wire. The commutator or collector consists of 49 parallel bars of copper, like the split-tube commutator of the other machines. The circuit of the armature runs from one bar of the commutator along one of the copper rods into a copper disc at the far end, crosses by this disc to the opposite rod, along which it comes back to the front end to another copper disc connected to the next bar of the commutator, and so on all round. This arrangement greatly reduces the wasteful resistance of the armature, and adds to the efficiency of the machine. The interior of the armature is made up of thin discs of iron strung upon the axis to intensify the magnetic action while avoiding the currents which would be generated wastefully (see Art. 401) in the mass of the metal were the iron core solid. There are

also 5 pairs of brushes at the commutator to diminish sparking. This machine has a very high efficiency, and turns 90 per cent of the mechanical power into electrical power. It is capable of maintaining 1300 of Edison's incandescent lamps (Art. 374) alight at one time. When driven at 300 revolutions per minute the current generated is about 900 ampères, and the electromotive-force 105 volts.

411. (c) Alternate-Current Machines.-In some dynamo

electric machines the alternately-directed currents generated by the successive approach and recession of the coils to and from the fixed magnet-poles are never commuted, but pass direct to the circuit. In a typical machine of this class invented by Wilde, the armature consists of a series of bobbins arranged upon the periphery of a disk which rotates between two sets of fixed electromagnets arranged upon circular frames, and presenting N and S-poles alternately inward. The alternatecurrent machine of Siemens is similar in design. Such machines cannot excite their own field-magnets with a constant polarity, and require a small auxiliary direct-current dynamo to excite their magnets. In another machine, devised by De Meritens, a ring-armature, resembling those of Pacinotti and Brush, moves in front of permanent steel magnets. In this machine the current induced in the circuit in one direction while the coils approach one set of poles is immediately followed by a current in the other direction as the coils recede from this set of poles and approach the set of poles of contrary sign. Alternate-current machines have also been devised by Lontin, Gramme, and others, for use in particular systems of electric lighting; as, for example, the Jablochkoff candle (Art. 374). In Lontin's machine, as in the more recent and much larger disk-dynamo of Gordon, the field-magnet coils rotate between two great rings of fixed coils in which the currents are induced. A recent form of alternate-current machine, designed by Ferranti, differs from the machines of Wilde and Siemens in the substitution of copper strips wound in zig-zag, for the set of rotating bobbins in the armature. This construction had previously been applied by Hopkinson and Muirhead.

411. (α) Compound-Wound Machines.— The field-magnets of a dynamo-electric machine are sometimes wound with two sets of coils, so that it can be used as a combined shuntand-series machine (see Art. 408). Such machines, when run at a certain “critical” speed, may be made to yield a constant current, or to work at a constant electromotive-force whatever the resistances in circuit. It is possible to attain either of these ends by combining, in one case a shunt-winding, in the other case a series-winding, with an independent magnetisation derived either from a permanent magnet or from a separatelyexcited field magnet.

CHAPTER XI.

ELECTRO-CHEMISTRY.

LESSON XXXVIII.—Electrolysis and Electrometallurgy.

412. In Lessons XIV. and XVIII. it was explained that a definite amount of chemical action in a cell evolves a current and transfers a certain quantity of electricity through the circuit, and that, conversely, a definite quantity of electricity, in passing through an electrolytic cell, will perform there a definite amount of chemical work. The relation between the current and the chemical work performed by it is laid down in the following paragraphs.

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413. Electromotive - force of Polarisation.— Whenever an electrolyte is decomposed by a current, the resolved ions have a tendency to reunite, that tendency being commonly termed "chemical affinity." Thus, when zinc sulphate (Zn SO) is split up into Zn and SO, the zinc tends to dissolve again into the solution by reason of its "affinity" for oxygen and for sulphuric acid. But zinc dissolving into sulphuric acid sets up an electromotive-force of definite amount; and to tear the zinc away from the sulphuric acid requires an electromotive-force at least as great as this, and in an opposite direction to it. So, again, when acidulated water is decomposed in a voltameter, the separated hydrogen

and oxygen tend to reunite and set up an opposing electromotive-force of no less than 1'47 volts. This opposing electromotive-force, which is in fact the measure of their "chemical affinity," is termed the electromotiveforce of polarisation. It can be observed in any watervoltameter (Art. 208) by simply disconnecting the wires from the battery and joining them to a galvanometer, when a current will be observed flowing back through the voltameter from the hydrogen electrode toward the oxygen electrode. The polarisation in a voltaic cell (Art. 163) produces an opposing electromotive-force in a perfectly similar way.

Now, since the affinity of hydrogen for oxygen is represented by an electromotive-force of 1'47 volts, it is clear that no cell or battery can decompose water unless it has an electromotive-force at least of 147 volts. With every electrolyte there is a similar minimum electromotive-force necessary to produce complete con

tinuous decomposition.

414. Theory of Electrolysis. Suppose a current to convey a quantity of electricity Q through a circuit in which there is an opposing electromotive-force E: the work done in moving Q units of electricity against this electromotive-force will be equal to E x Q. (If E and Q are expressed in "absolute" C.G.S. units, EQ will be in ergs.) The total energy of the current, as available for producing heat or mechanical motion, will be diminished by this quantity, which represents the work done against the electromotive-force in question.

But we can arrive in another way at an expression for this same quantity of work. For the quantity of electricity in passing through the cell will deposit a certain amount of metal: this amount of metal could be burned, or dissolved again in acid, giving up its potential energy as heat, and, the mechanical equivalent of heat being known, the equivalent quantity of work can be calculated. Q units of electricity will cause the depo