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 supportary 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.