The tungsten lamp has opened wonderful possibilities in the field of electric lighting by means of miniature lamps operated from batteries.

The tungsten lamp uses less than one third the amount of current required by the old-fashioned carbon filament lamps in producing the same candle power. This means that the battery will last three times as long.

Miniature lighting is one of the most practical phases of experimental electricity to which the experimenter can apply his time. Battery operated lamps will be found invaluable in many places for lighting dark corners, hallways, barns, garages, camps, bungalows, workshops, etc.

Tungsten Battery Lamps are made in a great many different styles and sizes.

The smallest sizes are used for flashlights, hand-lanterns, etc. These are not rated according to their candlepower, but to the voltage which they should be operated on. Flashlight lamps will be found satisfactory as a means of illuminating closets, cellars, stairways, etc., but do not give sufficient light for reading or any purpose of that sort. The light which they give varies from one to three candle-power according to the size of the lamp. They are made for 1.5, 2.8, 3.8 and 5.8 volts or respectively one, two, three and four dry cells. Flashlight lamps are provided with what is known as a miniature base only.

A very simple lighting arrangement which will prove exceedingly useful for illuminating a dark closet, a coal bin or the face of a clock, etc., consists of a 1.5 volt lamp, a single cell of dry battery and a suitable switch and wire. Such an installation may be made at a cost of less than seventy-five cents.

The lamp is mounted in what is known as a miniature porcelain base receptacle. The switch may most conveniently be the type termed a "wood base" switch. In case the lamp is installed for illuminating a clock, a "pear push" and a flexible cord may be substituted for turning the light off and on in place of the switch. It can then be suitably hung alongside of the clock or bed as desired.

The wire should not be smaller than No. 18 B. & S. Gauge or else it will offer too much resistance to the electric current and full benefit of the candle-power of the lamp will not be secured. Annunciator wire is preferable. It has good insulation and at the same time is not too large to be easily run through corners, cracks, etc.

The diagram in Figure 153 shows how to connect the lamp to the battery and switch. Figure 156 shows two dry cells connected in series for use with a 2.8 volt lamp.

When more illumination is desired than that furnished by flashlight lamps greater candle-power will be required. The following table shows the voltages and candle-powers of those lamps which are most suitable in such an instance:

• G6....4 volts 4 candle-power

• G6....6 volts 4 candle-power

• G8 6 volts 6 candle-power

They may also be operated from dry cells. If they are intended to be in use for other than a few minutes at a time it will be well to use a battery consisting of two sets of cells connected in series multiple. The method of connecting cells in series multiple has already been illustrated in Figure 35. This will lighten the strain on the battery and it will be found that two sets of batteries used in series multiple will last more than twice as long as one set used alone.

Six volt lamps of more than six candle-power are best operated on storage batteries because the current required is too great for dry cells to give efficient service in such a case.

The tungsten lamps made for automobiles are ideal for miniature lighting plants operated by storage batteries. The list below shows some of the sizes on a current of six to eight volts.

The bases are made of brass and come in four different styles, miniature, candelabra, single Ediswan and double Ediswan. The single and double Ediswan are used on automobiles because the construction of the base and the socket or the receptacle into which it fits is such that the lamp cannot jar loose.

The candelabra base is probably the type best adapted to miniature lighting elsewhere than on an automobile.

The letter "G" and the number in the first column indicates the size of the glass globe or bulb of the lamps.

You can compute the approximate amount of current drawn from a battery by a tungsten lamp, by dividing the candle-power by the voltage. The result is the current in amperes. For example, a 6 volt, 12 candle-power lamp will consume 12 divided by 6, or 2 amperes.

Storage batteries are rated by their output in ampere hours. An ampere hour is the amount of current represented by one ampere flowing for one hour. A forty ampere hour cell will deliver one ampere for forty hours, 4 amperes for ten hours or 5 amperes for eight hours. The ampere hour capacity of a storage cell divided by the amount of current being used will determine how long that current can be drawn before recharging is necessary.

The maximum amount of current it is safe to draw from a storage cell without impairing its life and efficiency is the maximum current which that cell can deliver continuously for eight hours. In order to determine this current simply divide the manufacturer's rating of the battery in ampere hours by eight. The answer is the maximum current you should require from the battery if you wish to obtain good service from it. For example, the maximum safe discharge rate of a 40 ampere hour battery is 40 divided by 8 or 5 amperes.

The method of computing the current required by a lamp and the method for determining the maximum safe discharge rate of a battery will enable you to figure the size of the smallest battery it is desirable to use in a lighting plant.

Suppose that you intend to use three 6 volt 12 candlepower lamps. Each lamp will require two amperes or a total of six when all three lamps are burning at the same time. Six amperes multiplied by eight ampere hours equals 48 ampere hours. This is the smallest size of battery which should be used. If it is larger, it will be advantageous, because it will supply the lamps a correspondingly longer time without recharging.

Figure 155 shows some of the sockets or receptacles which are on the market. The illustration to the left is a "flat base receptacle." It will be found most useful when the lamp is to be mounted directly on the wall or ceiling. The weatherproof type of socket may be secured with either a fibre or a porcelain shell.

The brass shell socket is the best wherever appearance counts. It is patterned after the standard Edison sockets and is made so that it will screw onto several different sizes and styles of brackets which may be mounted on either the ceiling or the wall. This type of socket is also made with a small switch built inside so that the lamp may be turned on or off directly at the socket.

The Wire used to connect the system will depend upon where the lamps are located. Annunciator or office wire as it is sometimes called will serve in most cases where the wires are to be run entirely indoors. The largest size made is No. 16 B. & S. Gauge. This is the size which should be used. Annunciator wire is made in various colors so that it can be more easily concealed.

If the wires are run outdoors, or somewhat exposed to the weather, as on a porch or in a barn, it is advisable to use weatherproof or rubber covered wire. Lead covered twin conductor is recommended for use aboard boats or wherever the wire is liable to abrasion.

If the lamps are larger than eight candle-power, or several are to be used on the same line, the wires should be larger than No. 16. When a battery is connected to a system of wires, the voltage at the end of the line is much lower than it is at the battery terminals. This is due to the resistance of the wire and is called "voltage drop." The lamps at the end of the line will not burn as brilliantly as those close to the battery. This can be partially overcome by using large wire.

The lamps may be either connected in series or in multiple. When a six volt battery is used, it will be necessary to connect them in multiple. Figure 159 shows how several lamps may be connected in multiple and controlled by one switch. Figure 160 shows the same lamps with three separate switches so that each lamp may be turned on or off individually.

It is possible to arrange a lamp and two switches so that it can be turned on or off at either switch independently of the other. This is a very convenient method of installing a light on a stairway or in a hall. One switch may be placed at the top of the stairs and the other at the bottom. A person going either up or down stairs can light the lamps ahead and turn it out as he passes the last switch regardless of its position at the other end.

The switches must be of the type called "two point" or "double throw single pole" and the levers must always rest on one of the contacts and never be left in between.