1388 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 
Il.—DUPLEX-DIPLEX TELEPHONY OVER WIRE CIRCUITS. 
Such has been the development of telephone engineering that at 
present any proposal which requires for its success the supplanting oi 
the present low-frequency battery system would be mostradical. It 
would surely be admitted that any plan which permits the present 
engineering telephone system to remain intact and superimpose 
thereon additional telephone circuits would possess cardinal advan- 
tages. Accordingly, the first preliminary experiments were directed 
to the inquiry as to whether or not it is possible to superimpose upon 
the minute telephonic currents now employed in telephony over wires, 
electric waves of ultra-sound frequencies without causing prohibitive 
interference with the battery telephone currents. Manifestly, this 
fundamental point can best be determined by experiments, at the 
generator itself, with the most sensitive part of the telephone equip- 
ment, viz, the telephone receiver. Accordingly, experiments were 
first conducted with various forms and types of telephone receivers in 
connection with local circuits at the generator. Such is the sensibility 
of the telephone receiver that it was thought possible that, although 
currents of frequencies entirely above audition were applied to the 
receiver from a dynamo as a source, there might be some frequency or 
frequencies from the operation of the apparatus which would be within 
the range of audition. Such was found, in fact, to be the case at cer- 
tain critical frequencies of the machine, but they were of no practical 
importance, as will be shown later. 
With a collection of telephone receivers ranging from about 50 to 
over 8,000 ohms and of a variety of designs, a series of tests was made 
under severe conditions to determine the above point. It was found, 
in general, that alternating currents of frequencies ranging from 30,000 
to 100,000 cycles per second, when coupled conductively, inductively, 
or ee ostatically to local circuits from the generator produced abso- 
lutely no perceptible physiological effects in the receivers, excepting 
only that at certain of the lower fr equencies a distinct audible note 
could be faintly heard in one of the receivers of about 250 ohms 
resistance. 
A search for the cause of this note Showed that it is due to a slight 
variation of the amplitude of the high-frequency current of the gen- 
erator, since no evidence of it could be detected on the battery ‘ele. 
phone side of the circuit. It appears to be caused by a very slight 
vibration of the rotor as a whole in the magnetic field of the generator. 
It was almost entirely removed by the simple device of opening out 
the stators, which increases the clearance and materially cuts down 
the flux of the machine. In practice it is a distinct advantage, how- 
ever, to have a trace of this note still left on the high-frequency eile of 
the éircuit, otherwise there is no ready means of | determining at the 
