Page:Incandescent electric lighting- A practical description of the Edison system.djvu/94

 the main block fixed to the back of the meter. From this latter end passes a wire^ which is secured in a binding post located at one end of a piece of metal passing behind the thermostat, to connect the two ends of the neutral wire entering the meter box at opposite sides, together. A lamp having been screwed into the socket, it is ready for operation.

The end of the brass-steel strip, opposite the screw on the lamp socket, is provided with a contact-point which will rest against the end of the screw under proper conditions; that is, when the temperature in the meter falls below a certain point, the brass on the brass-steel strip, contracting more than the steel, will cause the strip to curve, thus bringing the contact piece on its end against the lamp base screw, and, completing the circuit through the lamp, cause it to glow and disseminate the necessary heat.

It is a quality of sulphate of zinc to increase its resistance as it cools, and consequently the resistance of the bottles is