Miniature Tungsten Lamps.
—The wonderful light-giving properties of tungsten has made possible the use of miniature incandescent lamps for an almost infinite variety of usages. The miniature lamps are similar in action to other incandescent electric lamps except that they are operated on voltages lower than is used on commercial circuits. When used on commercial circuits, incandescent tungsten lamps of less than 10 watts capacity require filaments that are too delicate to withstand the conditions of ordinary use. The properties of tungsten are such that the passage of only a small amount of current is required to render the filament incandescent. In the case of a 110-volt circuit, a 10-watt lamp requires only 0.09+ ampere to produce the desired incandescence. It will be remembered that the watt is a volt-ampere and the 10-watt lamp will then require
110 volts × 0.09 + ampere = 10 watts.
Since 10-watt lamps are the smallest units that may be used on 110-volt circuits, their employment in smaller sizes must be such as will give more stable filaments. This is possible when the lamps are used at lower voltage. A 10-watt lamp on a 10-volt circuit will require an ampere of current.
10 volts × 1 ampere = 10 watts.
A filament suitable for an ampere of current is shorter and heavier than that of the 110-volt lamp and therefore furnishes a good form of construction. Still lower voltages may be used with filaments suited to the quantity of light desired.
In the case of battery lamps that are intended to operate on 1 or more volts, the filaments are made in size and length to suit the condition of action. In all cases the product of the volts and amperes give the capacity of the lamp in watts.
Miniature lamps are ordinarily marked to show the voltage on which they are intended to operate. A 6-volt battery lamp is intended to be used with a primary battery of four to six cells depending on the condition of usage, or three cells of storage battery, each cell of which gives 2 volts of pressure.