Calculations are greatly facilitated, and the value of tests can be ascertained quickly, if the constant of the brake is ascertained; then it will be simply necessary to multiply the number of revolutions and the weight at the end of the lever by such a constant, and the product gives the horse power, because, with a given Prony brake, the only variable quantities are the weight and the speed. All the observations, electrical and mechanical, are made simultaneously. The electrical horse power put into the motor is found by the well known formula C x E / 746; this simple multiplication and division becomes very tedious and even laborious if many tests have to be made in quick succession, and to obviate this trouble, and prevent errors, I have constructed a horse power diagram, the principle of which is shown in the diagram (Fig. 1).
Graphic representations are of the greatest value in all comparative tests. Mr. Gisbert Kapp has recently published a useful curve in the Electrician, by means of which one can easily compare the power and efficiency at a glance (Fig. 2).
The speeds are plotted as abscissae, and the electrical work absorbed in watts divided by 746 as ordinates; then with a series-wound motor we obtain the curve, EE. The shape of this curve depends on the type of the motor. Variation of speed is obtained by loading the brake with different weights. We begin with an excess of weight which holds the motor fast, and then a maximum current will flow through it without producing any external work. When we remove the brake altogether, the motor will run with a maximum speed, and again produce no external work, but in this case very little current will pass; this maximum speed is om on the diagram. Between these two extremes external work will be done, and there is a speed at which this is a maximum. To find these speeds we load the brake to different weights, and plot the resulting speeds and horse powers as abscissae and ordinates producing the curve, BB. Another curve,
e = B/E
made with an arbitrary scale, gives the commercial efficiency; the speed for a maximum external horse power is o a, and the speed for the highest efficiency is represented by o b. In practice it is not necessary to test a motor to the whole limits of this diagram; it will be sufficient to commence with a speed at which the efficiency becomes appreciable, and to leave off with that speed which renders the desired power.
I have now to draw your attention to a new motor of my own invention, of the weight of 124 lb., which, at 1,550 revolutions, gives 31 amperes and 61.5 volts at terminals. The mechanical horse power is 1.37, and the coefficient 373.
Ohms.
Armature resistance 0.4 w.
Field-magnet resistance 0.17 w.
Insulation resistance 1,500,000 w.
This motor was only completed on the morning before reading the paper; it could not, therefore, be tested as to its various capacities.
We have next to consider the principle of applying the motive power to the propulsion of a launch. The propellers hitherto practically applied in steam navigation are the paddle-wheel and the screw. The experience of modern steam navigation points to the exclusive use and advantage of the screw propeller where great speed of shaft is obtainable, and the electric engine is pre-eminently a high-speed engine, consequently the screw appears to be most suitable to the requirements of electric boats. By simply fixing the propeller to the prolonged motor shaft, we complete the whole system, which, when correctly made, will do its duty in perfect order, with an efficiency approaching theory to a high degree.
