Really Short Radio Waves


PROFESSOR NEIL H. WILLIAMS


ONE OF the noteworthy achievements by University of Michigan scientists in the past few years has been the production of very short electro-magnetic waves. This has been accomplished by Professor 
Neil H. Williams and his associates in the Department
 of Physics, notably Dr. C. E. Cleeton. When the radio 
engineer or operator speaks of short waves, he refers
 to wavelengths of 25 meters or thereabouts; Professor
 Williams, however, means really short waves when he
 uses the term; their lengths range between 6 millimeters
 and 3 centimeters. Though shorter waves of a spark type
 have been produced by others, Dr. Williams is the only 
physicist who has produced continuous radiations, useful 
for research purposes, with these diminutive wavelengths. 


A Prediction Fulfilled


This work all started from a prediction by Professor 
David M. Dennison that ammonia gas should absorb
 electromagnetic waves 1.5 cm. long. Smaller tubes, operat
ing on higher voltages and with stronger magnetic fields 
than had been used before, were constructed and suc
cessfully produced very short waves, and since the wave-
lengths are not far removed from those of infrared light, 
 optical methods were used in studying the results. The
 source of the rays was mounted at the focus of one large
 mirror; the rays were directed upon an echelette grating
 of aluminum, and thence reflected back to a second mir
ror and focused upon a crystal detector. To test the absorption of ammonia gas, a cell full of it was lowered in
 front of the receiving mirror and the absorption ob-
served. The results very closely agreed with Professor
 Dennison's prediction. 


Further experiments are now going on, with revised 
apparatus. Radiation of this sort can be guided through 
tubes, and consequently tubes have been substituted for 
the mirrors and grating for certain studies. The tubes
 may be filled with air or other substances; just now the 
interest is in the absorption of short waves by "heavy"
 ammonia gas—that is, ammonia in which the place of
 one of the ordinary hydrogens is taken by a heavy hy
drogen, or deuterium, atom. 


A New Field Of Research


The value of this work lies not simply in that it has 
produced new and curious playthings; it has opened a 
fresh region of the spectrum for study and has per-
mitted the physicist to observe how matter behaves when 
exposed to frequencies of a type which hitherto have been 
inaccessible. Already it has proved useful in checking 
the correctness of an hypothesis concerning the structure 
of the ammonia molecule. It may perhaps—though this 
is no prediction—be practically applied to radio com
munication over short distances by means of a directed
 beam, and also to the problem of seeing through fog.