SCIENCE AND INDUSTRY. 
question the fact of an electron emission independent of all causes except _ 
the nature of the metal and its temperature. His work made clear, also, 
the importance of a factor which had been overlooked by previous workers, 
viz.: the existence of a back-pressure or electromotive force due to the 
swarm of electrons surrounding the heated wire and tending to prevent the 
issue of further electrons. Just in the same way the drying of a wet string — 
in still air would be retarded by the presence around the string of an atmos- 
phere of water-vapour. And just as a current of air sweeps away this 
vapour and allows fresh vapour to issue, so does an electric field sweep away 
the electrons and allow a fresh swarm to take their place. 
Langmuir showed that, under proper conditions, exceedingly large currents 
could be obtained from an incandescent filament of tungsten at temperatures 
approaching its melting point (3,100° C.). The following table indicates the 
relation between electron emission and temperature :— 
TABLE I. 
Electron Emission from pure Tungsten. 
Absolute . Amperes 
Temperature. ~ per sq. cm. 
1,500° = i ..- 0.58 x 10-6 
S00 Sete we oy Be PAE eel Kr 
DET eRe % sf Pee OO15L 
BW eechts nea tae .. 0.18177 
5 OO ema a a paced eshy 
SHOTS iS +3 ae et Oli 
While Langmuir was pursuing this research his colleague:in the same 
laboratory, Dr. W. D. Coolidge, was simultaneously occupied with an inquiry 
into the characteristics and limitations of the ordinary X-ray bulb, in especial, 
with the attempt to use tungsten in place of platinum for the anti-kathode. 
He had come, in the course of this research, to the reluctant conclusion that 
no modification of this type of bulb was possible which would very materially 
increase its output without, at the same time, rendering the bulb unstable 
in operation. Langmuir’s results gave him a new inspiration. He saw the 
possibility of utilizing a pure electron current in the highest obtainable 
vacuum in place of the kathode rays of the older type of tube resulting 
' from the impact of positive ions on the kathode. From the outset success 
appeared certain, and in less than a year from the first attempt at construction ~ 
the first Coolidge X-ray bulbs were placed on the market. The construction 
of the tube may briefly be described: The source of the electrons is a small 
flat spiral of tungsten wire heated by a small storage-battery or from the 
low-voltage winding of a small transformer. Its temperature is thus con- 
-trollable by means of a variable resistance and ammeter in this circuit, a 
range of from four to six amperes varies the temperature from a dull red 
heat at which electron emission just begins to a bright white heat at which 
it is extremely copious. The anti-kathode or target (which is also the 
anode) is of tungsten either in the form of a solid forged block with plane 
face opposed to the filament or of copper in which a button of tungsten is 
centrally embedded. The tungsten spiral forming the kathode is centrally 
situated in a shallow cylindrical cup of molybdenum sheet metal forming 
part of a larger electrode, the whole being so designed as to focus the kathode 
stream of electrons on the face of the anti-kathode. The voltage is supplied 
534 
