Standard Voltage
We are assuming the farmer's plant to be, in 99 cases out of 100, the standard 110-volt, direct current type. Such a plant allows for at least a 10 per cent regulation, in voltage, up or down the scale; supplies for this voltage are to be had without delay in even the more remote parts of the country, and (being sold in greater volume) they are cheaper than those for other voltages.
There are two general exceptions to this rule as to 110-volt plants: (1) If the plant is located at a distance greater than a quarter of a mile from the house, it will be found cheaper (in cost of transmission line, as will be shown later) to adopt the 220-volt plant; (2), If the water supply is so meagre that it must be stored for many hours at a time, and then used for charging storage batteries, it will be found most economical to use a 30-volt plant. A storage battery is made up of cells of approximately 2 volts each; and, since more than 55 such cells would be required for a 110-volt installation, its cost would be prohibitive, with many farmers.
So we will assume that this plant is a 110-volt plant, to be run without storage battery. It will be well to make a chart, dividing the farm requirements into three heads—light, heat, and power.
Light
60 and 100 watt Mazda tungsten lamp. These lamps may be had in sizes from 10 to 500 watts (¼ scale)
The lamp of the future. A 1000 watt Mazda nitrogen lamp, giving 2000 candlepower (¼ scale)
Light is obtained by means of incandescent lamps. There are two styles in common use, the carbon and the tungsten lamp. It requires 3.5 to 4 watts of electricity to produce one candlepower in a carbon lamp. It requires from 1 to 1.25 watt to produce one candlepower in the tungsten lamp. The new nitrogen lamp, not yet in general use, requires only ½ watt to the candlepower. Since tungsten lamps give three times the light of the carbon lamp, they are the most economical to use in the city or town where one is paying for commercial current. But, in the country where water-power furnishes current for nothing, it will be found most economical to use the carbon lamp, since its cost at retail is 16 cents, as compared with 30 cents for a corresponding size in tungsten. A 60 watt carbon lamp, of 16 candlepower; or a 25 watt tungsten lamp, of 20 candlepower, are the sizes to use. In hanging lamps, as over the dining room table, a 100 watt tungsten lamp, costing 70 cents, and giving 92 candlepower light is very desirable; and for lighting the barn-yard, these 100 watt tungsten lamps should be used. For reading lamps, the tungsten style, of 40 or 60 watt capacity, will be found best. Otherwise, in all locations use the cheaper carbon lamp. Both styles have a rated life of 1,000 hours, after which they begin to fall off in efficiency. Here again, the farmer need not worry over lack of highest efficiency, as a lamp giving only 80 per cent of its rated candlepower is still serviceable when he is not paying for the current. With care not to use them at voltages beyond their ratings, lamps will last for years.