DISTRIBUTION AT CONSTANT PRESSURE.

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constant, but the current delivered into the mains must be the greater the greater the number of motors at work. We have thus to distinguish between distribution at constant pressure and distribution at constant current. The first system is represented-though as yet on a more restricted scale than was anticipated by the promotersby the Edison system in New York and Berlin, the latter by the Thomson-Houston system.

Electric Distribution of Energy at Constant Pressure.

We must now inquire into the theoretical conditions of this case. It will be evident at the outset that for economical reasons any attempt to obtain constancy of speed by the use of artificial resistances can only lead to a partial and not very satisfactory solution of the problem, and had better not be made if other means are at hand. This, happily, is the case in the present instance. We have two means by which we can without waste regulate the power of the motor to the work and yet keep it running at a constant speed. First, we may apply a mechanical device by which the power of the electric current is cut off in proportion as the work is cut off, and, secondly, we may apply an electrical device in the shape of special winding of the field magnets of the motor by which the torque exerted by the armature is automatically regulated so as to correspond to the mechanical load. As regards the first system, which applies equally well to the distribution at constant pressure, and to that at constant current, Professors Ayrton and Perry have in a paper on Electro-Motors and their Government1 shown how this can be done. They say: "The method of cutting off the

"Journal of the Society of Telegraph Engineers and Electricians," No. 49, vol. xii. 1883.

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power as hitherto employed has this serious defect, that instead of the power cut off being directly in proportion to the work cut off, the arrangements have been such that either all power was cut off or none, so that the motion of the motor was spasmodic, just as in an ordinary gas-engine, which suffers from the same defects, that full charge of gas or no charge are the usual only alternatives. An electro-motor governor of this type, which may be called a 'spasmodic governor,' consists merely of a rotating mercury cup into which dips a wire, which makes in this case contact with the mercury, and so completes the circuit when the speed is slow, but which, on account of the hyperbolic form assumed by the surface of the mercury as the speed rises, ceases to dip into the mercury at high speeds, and so breaks contact." Later on the inventors say: "The first improvement we made in governing consisted in replacing the 'spasmodic governor' by a 'periodic governor.' With our periodic governor the power is never cut off entirely for any length of time, but in every revolution power is supplied during a portion of the revolution, the proportion of the time in every revolution during which much power is supplied to the time during which less is supplied depending on the amount of work the motor is doing. Our periodic governor, then, differs from the spasmodic governor in the same way that a good loaded steam-engine governor differs from the ordinary governor of a gas-engine. One of the ways of effecting this result is as follows: a brush, A, Fig. 64, lies on the rotating piece, B K, the cylindric surface of which is formed of two conducting portions connected with one another through any resistance, and the brush, A, is moved along the cylinder B K under the action of the governor balls. When the brush A is touching the contact part B, the

PERIODIC GOVERNOR.

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motor is receiving current directly, but when it rests on the part K, the motor receives current through the resistance which is interposed between B and K. If the governor balls fly out, the brush is moved along B, K, so that there is contact with K during a greater part of the revolution than before; and if the governor balls come together, the speed of the motor being too small, the brush is moved in the opposite direction so that it makes

contact with B for a longer time during each revolution. If the motors are in series, we arrange that the periodic governor shunts the current periodically, instead of introducing resistance. In this case the connections are as follows: B is made of wood, while K is made of metal. K is connected to one end of a shunt coil, the other end of the shunt being connected to one of the terminals of the motor and A is connected to the other terminal of the motor. If, then, A rests on B, the shunt is inoperative and all the current passes through the motor; whereas, if it rests on K, the shunt is in operation, and part of the current only passes through the motor." It

will be seen that both these governors invented by Professors Ayrton and Perry, have partially, at least, the fault of depending on artificial resistances whether they be used for parallel or series work. The loss of energy thus occasioned can be reduced by making the resistance high for parallel, and low for series work, and on purely theoretical grounds it could even be entirely prevented by making the resistance infinite, that is, breaking the circuit altogether during a portion of each revolution when working in parallel. But this would produce an unequal turning force, and would also entail destructive sparking between the brush, A, and the contact pieces B and K. Even if the resistance between B and K or that of the shunt coil between K and one terminal of the motor is fairly low, there seems to be some sparking; for the inventors say in their paper that with any such governors it is difficult to entirely prevent sparking, and that on this account motors wound so as to be self-regulating without any mechanical device are preferable.

Broadly speaking, the self-regulating motor is the converse of the self-regulating dynamo wound for constant pressure. In a properly compounded dynamo the electromotive force or pressure at the terminals must remain constant, although the resistance of the external circuit may vary between wide limits, causing an inversely proportional variation in the external current. The power required is approximately proportional to the current. The machine works, therefore, under these conditions: Speed constant-Electro-motive force constant-Current variable-Power required to drive the machine also variable, but proportional to current. Now, in a self-regulating motor the conditions are:-Electro-motive force constant-Speed constant-Power variable-Current re

SELF-REGULATING MOTOR.

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quired to drive the motor also variable, but proportional to power.

It has already been pointed out that in a general way dynamo and electro-motor are convertible terms; and although there are cases when it is impracticable to work a motor as a dynamo, it is always perfectly easy to work a dynamo as a motor. From this general convertibility it is reasonable to expect that a properly compounded dynamo can without any alteration in the connection between its field magnet coils and armature, be used as a selfregulating motor, the only condition being that it shall be supplied with current at a constant electro-motive force. When speaking of a self-regulating motor in the sense that its speed of rotation shall automatically be kept constant whatever variation might occur in the load or mechanical resistance which the armature of the motor has to overcome, it must be understood that this refers only to such cases where the load varies between zero and a maximum not beyond the capability of the motor. If we throw an excess of load on to the motor, it will pull up or slacken speed, and thus cease to be self-regulating, just as the electro-motive force at the terminals of the best compound-wound dynamo will be lowered if we allow an excess of current to flow. But within a reasonable limit of load in the case of the motor, and a reasonable limit of current in the case of the dynamo, both machines can be made self-regulating, and this result is obtained by the same means, that is to say, the same winding which will make the dynamo give a constant electro-motive force, will make the motor run at a constant speed. This result might be expected on the ground of the general convertibility of these machines, but since it is of great practical importance, special proof