The incandescent lamps are usually rated in light-giving properties by their value in horizontal candlepower. This represents the mean value of the light of the lamp which comes from a horizontal plane passing through the center of illumination and perpendicular to the long axis of the lamp. Candlepower in this connection originally referred to the English standard candle which is made of spermaceti. The standard candle is 0.9 inch in diameter at the base, 0.8 inch in diameter at the top and 10 inches long. It burns 120 grains of spermaceti and wick per hour. This candle is not satisfactory as a standard because of the variable conditions that must surround its use. The American or International standard is equal to 1.11 Hefner candles. The Hefner candle (which is the standard in continental Europe and South American countries) is produced by a lamp burning amylacetate. This lamp consists of a reservoir and wick of standard dimensions which gives a constant quantity of light. The light from this lamp has proven much more satisfactory as a means of measurement of light than the English standard and therefore its use has been very generally adopted.

The light given out by an incandescent lamp is not the same in all directions. In making comparisons it is necessary to define the position from which the light of the lamps is taken. The horizontal candlepower affords a fairly exact means of comparing lamps which have the same shape of filament, but for different kinds of lamps it does not give a true comparison. The spherical candlepower is used to compare lamps of different construction as this gives the mean value at all points of a sphere surrounding the lamp. The candlepower is measured at various positions about the lamp with the use of a photometer, and the mean of these values is taken as the mean spherical candlepower.

At their best, carbon-filament lamps require in electricity 3.1 w.p.c. (watts per candlepower). As the lamp grows old the number of watts per candle power increases, until in very old lamps the amount of electricity used to produce a given amount of light may become excessively large. According to a bulletin issued by the Illinois Engineering Experiment Station on the efficiency of carbon-filament incandescent lamps, the amount of electrical energy per candlepower varied from 3.1 w.p.c, when new, to 4.2 w.p.c, after burning 800 hours.

A common practice in the use of carbon-filament lamps is to consider that the period of useful life ends at a point where the amount of electricity, per candlepower, reaches 20 per cent. in excess of the original amount. This point (sometimes termed the smashing point) would be reached after 800 working hours, according to the Illinois Station, and at about 1000 hours as stated by the bulletins of the General Electric Co. If a carbon-filament lamp burns for an average period of 3 hours a day for a year, it ought to be replaced.

The Edison screw base as shown in Fig. 217 is now generally used in all makes of incandescent lamps for attaching the lamp to the socket. When screwed into place this base forms in the socket the connections with the supply wires, to produce a circuit through the lamp. One end of the filament is attached to the brass cap contact, the opposite end connects with the brass screw shell of the base. When the current is turned on, the contact made in the switch is such as to form a complete circuit between the supply wires; the voltage sending a constant current through the lamp produces a steady incandescence of the filament.

In Fig. 218 is shown a carbon-filament lamp attached to an ordinary socket. The lamp base and socket are shown in section to expose all of the parts that comprise the mechanism. The insulated wires of the lamp cord enter the top of the socket and the ends attach to the binding screws A and B, which are insulated from each other and form the brass shell which encases the socket.

The lamp base is shown screwed into the socket, the brass cap contact F making connection at G; the screw shell joins the socket at D. To the key S is attached a brass rod R, on which is fastened E, the contact-maker. The rod R passes through a sup-portary frame which is secured to the lamp socket at G. As shown in the figures the piece E makes contact with a brass spring attached to A, and this completes a circuit through the filament. The brass cap contact of the lamp base makes connection at one end of the filament H, the other end of the filament K is attached to the brass screw shell of the base, which in turn connects with the screw shell of the socket and this shell is connected with the piece containing the binding screw B by the rod C to complete the circuit. When the key S turns, the contact above E is broken and the lamp ceases to burn.

Fig. 118 shows the use of an adapter that is sometimes encountered in old electric fixtures, the use of which requires explanation. Mention has already been made of the various forms of lamp sockets in use before the Edison base became a standard. In order to use an Edison lamp in a socket intended for another form of base an adapter must be employed to suit the new base to the old socket. In the figure the piece P1, is the adapter. This is intended to adapt the standard lamp base to a socket that was formerly in use on the Thompson-Houston system of electric lighting. The adapter is joined to the old socket by the screw at G and the circuit formed as already described.