because the armature never moves more than one third of a revolution, but we now wish to examine an instrument in which

ding posts to the wire of the moving coil? I will lift the cover off this instru

2-candle-power lamp. According to the ammeter, whi

e and the other to the other plate, and the metal strips brush along over the surfaces of the plates. (That is why they are called "brushes," I said.) And the brushes slide from one plate to the other each time the armature makes half a

zle to you. Every machine, while it is being driven by an electric current as an electric motor, is, at the same time, acting as a dynamo to generate a current in the opposite direction. Notice in the third place that this is a shunt-wound instrument. The current which is sent into the instrument divides, and part of it goes through the field, while part goes through the armature. Motors, as well as dynamos, are either shunt-wound or series-wound. But notice finally that the axle on which

o hours or two amperes for half an hour. In any case this product is called one ampere hour. But the words printed upon the dials of this instrument are not ampere hours, but watt hours and

g.

we have a mill pond, (Fig. 17, A). We construct a penstock p and install a water-wheel, S, to operate a mill. Our business increases and we install more machinery in our mill and must have more power to run it. We have two ways of getting it, (1) we may lengthen our wheel and enlarge our penstock so that a greater weight of water will fall upon the wheel, or (2) we may lengthen our penstock and move the wheel f

s up. That is, seven amperes at 110-volt pressure give one horse-power. In the office building across the street where they use a 220-volt current 3? amperes are required to take us up stairs at the same speed. It is necessary that the same amount of coal be consumed to furnish the horse-power of energy whether we supply it by means of seven amperes at 110 volts or 3?

wer for an electric motor, we must have a definite number of watts. We may choose whether

g.

he filament in the lamp resists the passage of the current. It gets quite hot and gives forth as much light as thirty-two candles. Its resistance is just such that 110 volts of pressure send one ampere through it. We will now take the reading of the wattmeter, note the time and read it again later. One hour later its index showed that 110 wat

h by Hele

tme

ut how much electricity has been used for electric lights, for electric ventilating fans, for electric elevators, for electric ovens, and electric irons in the school of household arts, for electric motors

next question that they would bring to me. Knowing, however,