Fig. 221.—Miniature lamp bases.
The term miniature lamp applies more particularly to the base than to the voltage or candlepower. The style of base is characteristic of the service for which the lamp is designed rather than the size or number of watts consumed. There are two general styles of bases: the screw type of the Edison construction of which there are two sizes; and the bayonet type of which there are two styles of construction.
Bases for miniature lamps are made in form to suit the conditions of their use. The styles at present are shown in Fig. 221. Of these the screw bases at the left are those attached to small flash-lamp bulbs and others of the smaller sizes of lamps. The two at the right of the figure are the bayonet style used under conditions not suited to the screw contact. In the case of automobile lamps and in places where vibration will cause loss of contact the bayonet base is generally in use. The lamp is held in place by the projecting lugs that engage with openings in the socket and kept in place by the pressure of a spring. The contact with the lamp filament is made by two terminals that make connection directly with the terminals of the lamp filament. The single contact base is kept in place similarly to that of the other but makes a single contact at the end of the socket while the other but makes a single contact at the end of the socket while the circuit is completed through the pressure exerted between the projecting lugs and the socket.
Effect of Voltage Variations.
—Voltage variation may be temporary, due to changing load in the circuit, or in constantly overloaded circuits the voltage may be constantly below normal. The change in electric pressure affects in a considerable degree the amount of light given by the lamp. As an example, a 5 per cent. drop from the normal voltage will cause a decrease of 31 per cent. in the amount of light given. This means that if a lamp is working on a circuit of 110 volts and the voltage from any cause were to drop to 104½ volts, the light would decrease 6.8, almost 7 candlepower. Drop in voltage may also be due to the resistance of wires that are too small for the service. Lamps attached to such a circuit will constantly burn dim.
Turn-down Electric Lamps.
—The ordinary incandescent lamp lacks the flexibility of gas and oil lamp, in that the amount of light cannot be varied at will. This feature is attained in the electric turn-down lamp either by resistance added to the lamp circuit or by the use of two separate filaments in a single globe; one of ordinary lamp size and the other of such size that it consumes only a fraction as much energy as the normal lamp.
Fig. 222.—Sectional view of a “turn-down” lamp socket.
Turn-down lamps of the latter form are made in several styles, the chief points of difference being in the method of changing the contact from the high-to the low-power filament. In Fig. 222 a sectional view shows the “pull-string” form of lamp in which the parts are exposed. The long filament H and the smaller one L represent two individual lamps of different lighting power. The change in light is made from one to the other by pulling the string which is attached to a switch in the socket and which changes the contact to send the current through the filament giving the desired amount of light. The figure shows a carbon-filament lamp, but tungsten lamps are made to accomplish the same purpose. The difficulty of manufacturing a 1-candlepower tungsten lamp for direct operation on a 110-volt circuit requires the filaments to work in series. The figure is arranged on the same plan as for a tungsten lamp.