134 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 
There are, therefore, two possible solutions to the problem of 
multiplex telephony and telegraphy upon this principle by electric 
waves, based upon the unalterable characteristics of the human ear, 
viz, by employing (1) electric waves of infra sound frequencies, and 
(2) those of ultra sound frequencies. One great difficulty in design- 
ing generators of infra sound frequencies is In securing a pure sine 
wave, as otherwise any harmonic of the fundamental would appear 
within the range of audition. Furthermore, the range of frequencies 
is restricted, and the physical dimensions of the tuning elements for 
such low frequencies would have a tendency to become unwieldy. 
The electromagnetic spectrum at present extends from about four 
to eight periods per second, such as are employed upon, ocean cables, 
to the shortest waves of ultra-violet light. In this whole range of 
frequencies there are two distinct intervals which have not as yet 
been used, viz, frequencies from about 3X10” of the extreme infra- 
red to 5X10", which is the freqeuncy of the shortest electric waves 
yet produced by electrical apparatus, and from about 80,000 to 100,000 
cycles per second to about 15,000 to 20,000 cycles per second. The 
upper limit of this latter interval represents about the lowest frequen- 
cies yet employed for long-distance wireless telegraphy. 
Within the past few years generators have been developed in the 
United States giving an output of 2 kilowatts and above at a fre- 
quency of 100,000 cycles per second, and also capable of being ope- 
rated ea at as low a ee as 20,000 cycles per second. 
Furthermore, these machines give a practically pure sine wave. 
The necessary conditions for telephony by electric waves guided 
by wires are an uninterrupted source of sustained oscillations and 
some form of receiving device which is quantitative in its action. In 
the experiments described in multiplex telephony and telegraphy it 
has been necessary and sufficient to combine the present engineering 
practice of wire telephony and telegraphy with the engineering 
practice of wireless telephony and telegraphy. 
The frequencies involved in telephony over wires do not exceed 
1,800 to 2,000, and for such frequencies the telephonic currents are 
fairly well distributed throughout the cross section of the conductor. 
As the frequency is increased the so-called “skin effect’? becomes 
noticeable, and the energy is more and more transmitted in the ether 
surrounding the conductor. 
It has been found possible to superimpose, upon Ane ordinary tele- 
phonic wire circuits now commercially used, electric waves of ultra 
sound frequencies without producing any harmful effects upon the 
operation of the existing telephonic service. Fortunately, therefore, 
the experiments described below are constructive and additive, 
rather than destructive and supplantive. 
