B

B'

with three separate circuits in 1878. If two equal alternate currents, differing in phase by one-quarter of a period, are properly combined, they can be made to produce a rotatory magnetic field. And in such a rotatory field conductors can be set rotating, as was first suggested by Baily in 1879. Consider an ordinary Gramme ring (Fig. 264) wound with a continuous winding. If a single alternating current were introduced at the points AA' it would set up an oscillatory magnetic field, a N pole growing at A, and a S pole at A', then dying away and reversing in direction. Similarly, if another alternate current were introduced at BB', it would produce another oscillatory magnetic field in the BB' diameter. If both these currents are set to work but timed so that the BB' current is period behind the

Fig. 264.

AA' current, then they will combine to produce a rotatory magnetic field, though the coil itself stands still. This is quite analogous to the wellknown way in which a rotatory motion, without any dead points, can be produced from two oscillatory Fig. 265. motions by using two cranks at right angles to one another, the impulses being given period one after the other. The above combination is called a di-phase system of currents. If the BB' current is

B

period later than the AA' current, the rotation in Fig. 265 will be right-handed. Another way of generating a rotatory field is by a tri-phase system* (or so-called "dreh-strom ") of currents. Let 3 alternate currents, differing from one another by period (or 120°) be led

*Tri-phase currents were used in the famous Frankfort transmission of power in 1891. See Art. 447.

into the ring at the points A B C. The current flows in first at A (and out by B and C), then at B (flowing out by C and A), then at C (out by A and B), again producing a revolving magnetic field. This is analogous to a 3-crank engine, with the crank set at 120° apart.

There are several ways of combining the circuits that receive the currents of the various phases. For example, the windings of Fig. 264 might be divided into four separate coils, each having one end joined to a common junction, and the four outer ends joined respectively to the four line wires. Or the windings of Fig. 265 might be arranged as three separate coils, each having one end joined to a common junction, and with the three outer ends joined respectively to the three line wires. Such arrangements would be called star groupings, as distinguished from the mesh groupings of the cuts. Also the coils, in whichever way grouped, need not be wound upon a ring. The two-phase coils of Fig. 264 might be wound upon four inwardly-projecting pole-pieces; and the three-phase coils of Fig. 265 might be wound upon three inwardly-projecting pole-pieces. Or in larger multipolar machines a three-phase set of coils might be arranged upon a set of six, nine, twelve, or more projections, in regular succession.

For generating two-phase (or three-phase) currents the alternators must be designed with two (or with three) separate sets of windings in the armature; these separate sets of windings being so spaced out as to come into inductive operation in regular succession. There will thus be two (or three) independent circuits of equal voltage, which may be then connected up in either a star-grouping or a mesh-grouping as described above. To transmit the two-phase currents four line-wires are usually employed. For transmitting three-phase currents three wires suffice.

486. Properties of the Rotatory Field - Asynchronous Motors. In such rotating maguetic fields masses of metal at once begin to rotate. A magnet or mass of iron,

pivoted centrally, can take up a synchronous motion, but may require to be helped to start. Any pivoted mass of good conducting metal, such as copper, will also be set in motion, and will be self-starting, but will not be synchronous. In such a centred mass, or rotor, eddy-currents are set up (just as in Arago's rotations, Art. 457), which drag the metal mass and tend to turn it. The strength of these currents in the rotating part depends on the relative speed of the field and the rotor. If the rotor were to revolve with speed equal to the revolving field, the eddy-currents would die away, and there would be no driving force. The rotor actually used in such motors consists of a cylindrical core built up of thin iron disks, over which is built up a sort of squirrel cage of copper rods joined together at their ends into a closed circuit. In some forms (designed by Brown) the rods are inserted in holes just below the surface of the core. The rotor need not have any commutator or slip-rings, and is entirely disconnected from any other circuit. It receives its currents wholly by induction. Such asynchronous motors start with considerable torque (or turning moment) and have a high efficiency in full work. Similar motors for use with ordinary or single-phase alternate currents are now in use. To start them it is necessary to split the alternate current into two currents differing in phase. This is done by the use of a divided circuit, in the two branches of which different reactances are introduced. If in one branch there is a choking coil to offer inductance, the current in that branch will be retarded; if in the other there is a condenser, the current in this branch will be accelerated in phase. Combining these two currents a rotatory field is produced for starting the movement. When once the motor has started a further turn of the switch simply puts on the alternate current, as at AA' in Fig. 264, and it continues to be driven, though the impulse is now only oscillatory.

CHAPTER XI

ELECTRO-CHEMISTRY

LESSON XLVII. — Electrolysis

487. Electromotive-force of Polarization. - The simple laws of definite chemical action due to the current having been laid down in Lesson XIX. it remains to consider the relations between the chemical energy and its electrical equivalent. 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 (ZnSO1) is split up into Zn and SO, the zinc tends to dissolve again into the solution, and so spread the potential energy of the system. 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 electronotive-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 electromotive-force of polarization. It can be observed in any water voltameter (Art. 243) 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 polarization in a voltaic cell (Art. 175) 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 at ordinary temperatures unless it has an electromotive-force of at least 1.47 volts. With every electrolyte there is a similar minimum electromotive-force necessary to produce complete continuous decomposition.

488. 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 × Q. (If E and Q are expressed in absolute" C.G.S. units, Ex Q 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.

66

Q

But we can arrive in another way at an expression for this same quantity of work. 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. units of electricity will cause the deposition of Qz grammes of an ion whose absolute electro-chemical equivalent is z. [For example, z for hydrogen is 0001038 gramme, being ten times the amount (see Table in Art. 240) deposited by one coulomb, for the coulomb is of the absolute C.G.S. unit of quantity.] If H represents the number of heat units evolved by one gramme of the substance, when it enters into the combination in question,