New Developments in Science 583 



electric-light bulb. It is coated on the inside with a layer of mer- 

 cury, to keep out light, and a layer of potassium, which is very 

 sensitive to light. On one side of the bulb is a small, round "win- 

 dow" of clear glass, without either the mercury or the potassium 

 coating. In the center of the bulb is a filament consisting of an 

 upright metal ring connected by a wire to the base of the cell. 



In use, the photoelectric cell is connected in a circuit, with the 

 filament joined to the positive pole of a source of electricity and 

 the potassium lining of the bulb joined to the negative pole. A 

 special lamp is adjusted to send a beam of light through the clear 

 glass "window" into the cell. When the lamp is not turned on, 

 and the cell is in darkness, no current flows between the filament 

 and the potassium lining. But when a beam of light from the lamp 

 enters the cell through the round window of clear glass, it causes 

 the potassium lining to give off electrons, and a current of elec- 

 tricity passes between the filament and the potassium lining. The 

 stronger the beam of light, the more powerful is the current of 

 electricity which flows through the circuit. 



Today, the photoelectric cell has a great many uses. Astrono- 

 mers use it to measure the intensity of the light coming to us 

 from the different stars, for it is so sensitive that it can detect 

 differences in brightness not apparent to the human eye. It is 

 used to count objects of all sorts. For example, on one side of the 

 entrance of the Holland Vehicular Tunnel, which joins New York 

 and Jersey City, there is a photoelectric cell used in an electric 

 counting machine. On the opposite side of the tunnel is a lamp 

 which sends a beam of infra-red light to the cell. As long as the 

 light strikes the cell, a continuous current of electricity flows 

 through the circuit. Every automobile that passes through the 

 tunnel passes between the lamp and the cell, thus cutting off the 

 beam of light, interrupting the current, and operating the count- 

 ing machine. The photoelectric cell is used in burglar alarms. In 

 one such device a special lamp sends an invisible beam of infra- 

 red light across the front of a safe or across a doorway to a 

 hidden photoelectric cell. When a person passes between the lamp 

 and the cell, he cuts off the infra-red light and interrupts the 

 electric current flowing through the cell. This rings a burglar 



