generating dynamo, and the current is forced back into it. For the time being the motor acts as a generator, the energy stored mechanically in its revolving armature being returned to the circuit in the form of current. This reverses the polarity of the compound dynamo (its shunt coils being weak, as stated above), and now both the generator and the armature of the motor are working in series, the generator assisting instead of opposing the current started by the motor. At this moment we have the following state of things:-The field magnets of the motor have just attained their maximum of magnetization with their original polarity; the polarity of the generator has been reversed, and an excessive current, in an opposite direction to that which produced motion, flows through the armature of the motor. Consequently the latter is quickly brought to rest, and started backward at a high speed. It now opposes a certain counter-electromotive force to the current from the generator, but it is not an increasing force as before. It is a decreasing one, because the original excitation of the motor field magnets is gradually vanishing, by reason of the reversal of polarity in the main leads, from which these shunt coils are fed. Just as it took a certain appreciable time of several seconds for the magnets to become excited, so does it take time for them to lose their magnetism. Eventually there arrives a moment when all the original polarity in these magnets has vanished, and when, therefore, the force impelling the armature to run backward has also ceased, though there is still an excessive current passing through it. A moment later the armature comes to rest, and begins to run forward again at a high rate of acceleration, when the whole cycle of phenomena just described is repeated, but this time with a current in the

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reverse direction to the first. The third cycle will start with a current in the same direction as the first, the fourth cycle will start with an opposite current, and

so on."

A similar phenomenon was observed by M. GérardLescuyer, who used a Gramme series-wound dynamo as a generator, and a magneto machine as a receiver. He called the phenomenon an electro-dynamic paradox, and a description of it will be found in "The Engineer" of Sept. 17, 1880.

CHAPTER VI.

Classification of Systems according to Source of Electricity-Transmission at Constant Pressure-Motors mechanically_governed-Self-Regulating Motors Transmission at Constant Current-Difficulty of Self-Regulation -Motor for Constant Current made Self-Regulating Application to Transmission over large Areas-Continuous Current TransformatorTransmission between two Distant Points-Loss of Current by LeakageTheory-Commercial Efficiency-Conditions for Maximum Commercial Efficiency Self-Regulation for Constant Speed-Practical Example.

It will be necessary to distinguish between different systems of electric transmission of energy, according to the source of electricity. An almost endless variety of cases may present themselves in different applications of electrical transmission, but three systems are of special interest, because most frequently occurring in practice. These are the following:

1. Transmission of energy from primary or secondary batteries at short distances to one motor only.

2. Transmission of energy from one or several dynamos to a number of motors placed upon the same circuit, but working independently of each other.

3. Transmission of energy between two distant points by means of one generator and one motor.

We may also make another classification according as the motors are intended for a constant or variable load. or a constant or variable speed. Generally speaking, the systems of transmission coming under heading 1) are not required for a constant load, nor is it of any great impor

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tance that the speed should remain constant under a variable load. We shall not enter into a minute description of these cases here, as the investigation of electric tramways and railways, worked by accumulators, will afford ample opportunity of entering into details.

System 2) is that presenting most difficulties on account of the condition that all the motors must be independent of each other. The case is further complicated by the requirement that each motor should run with the same speed when empty or loaded. A moment's consideration will show that the last condition is an absolute necessity if we would make the electric transmission of energy of real practical use to small domestic industries. The artisan or small manufacturer would have his motor connected to a common system of service leads, and whenever he required power he would switch the current on to his motor. In doing so he must not disturb any other work which, at the same time, may be done elsewhere from the same service mains, such, for instance, as lighting or working other motors; and further, his motor should always run at the same safe speed, whether it is giving him little or much mechanical energy. Most operations requiring the use of tools as turning, planing, &c., can only be properly performed at a certain fixed rate of speed, and the machinery must be kept going at that rate at all times.

System 3) presents difficulties of a different nature. Since we have to deal only with one generator and one motor, it is easier to make each fit the other, and as a rule the load is fairly constant, so that regularity of speed is not difficult to obtain. In this case the difficulty lies more in the necessity of proper insulation of line and machinery. Generally speaking, the system is required

for long distance transmission, and to obtain an economical arrangement, both as regards first cost and commercial efficiency, the use of a high electro-motive force is necessary. This entails some danger to human life, and some difficulty in maintaining an efficient insulation. Both these points can, however, be satisfactorily dealt with, if proper care is used in the design and execution of the work. As regards the danger to human life involved in the use of electric currents of high pressure, this is generally greatly overrated. It is quite possible for a man who with both hands should touch the positive and negative wires in a non-insulated part, to be killed or severely injured if the pressure is over two or three thousand volts, but the accident can be rendered almost impossible if due precaution is taken. A circular saw if only lightly touched whilst revolving will cut a man's finger off, and what can be more dangerous than a pair of powerful spur wheels? Yet we have found means of protecting life very effectually from destruction by purely mechanical means, and shall, without much difficulty, find means for protecting it from the electrical danger.

System 2) is best described as an electrical transmission and distribution of energy from one central station to several distant points. Now this distribution can be made on the parallel or on the series system. In the first case the electro-motive force (or pressure) between the positive and negative mains must be kept constant, and the motors are connected all in parallel from the mains ; in the second case the current passing through the mains must be kept constant, and each motor, when at work, is traversed by the same current. The pressure at the station must be the greater the greater the number of motors at work. In the first case the pressure is kept