"The Gatehouse Lamp.
The Gatehouse lamp, as employed by the Pilsen, Joel, and General Electric Lighting Company, has a f1lament of peculiar construction. In all other incandescent lamps the resistance of the filament when cold is considerably greater than when the lamp has been burning and the filament has had time to become heated. This is due to the fact that the resistance of carbon decreases with a rise in temperature. Thus the resistance of an 8-candle-power Edison lamp is 112 ohms when cold and only 70 ohms when hot. If, then, a current of too great strength be passed by accident through an ordinary incandescent lamp, the filament, to use a figure of speech, courts its own destruction by offering less resistance to the current the stronger the latter becomes. Now, while carbon decreases in resistance with a rise in temperature, with platinum the exact opposite is the case. In the Gatehouse lamp the filament is composed of three consecutive pieces joined end to end, the middle section being composed of carbon, while the end pieces are spirals of thin platinum wire. When the proper amount of current is passing through the lamp, the carbon alone is heated, the platinum being only slightly warm. Should, however, the current increase in strength, the platinum is heated, the resistance of the entire filament increased, and the strength of the current consequently reduced. By means of this compound filament the patentees hope to make the lamp more durable, since the carbon filament is protected from being heated to too high a temperature.
The filament is inclosed in the usual manner in a pear- shaped glass globe containing a high vacuum. The Gatehouse lamp is made in sizes to give a light of from 5 to 50 candles. With the ordinary 2o-candle-power lamp the current required is said to be 1-5 amperes with an electromotive force of 60 volts."