174 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
the curve depends upon the sum of the decrements of the sending and 
receiving circuits. By the term “ decrement” of a circuit is meant 
the logarithm of the ratio of the amplitudes of two successive oscil- 
lations in the train. 
To obtain very sharp tuning we have therefore to employ either 
undamped oscillations or very feebly damped oscillations in the 
transmitter, and also a receiving circuit in which there is as little 
dissipation of energy by resistance and other causes as possible. It 
is then possible to cause a change of even less than one-half of 1 per 
cent, or 5 parts in 1,000 in the wave length of the received waves 
to cease to actuate the receiver. This means that we can distinguish 
between two waves 1,000 and 1,005 or 1,010 feet in length, respec- 
tively, and that our receiver may be tuned to respond to one and not 
to the other. The persistent or undamped oscillations created by 
the arc transmitters have, therefore, an advantage in this respect 
-over spark transmitters, in that the damping or decrement of the 
transmitter is less; but it should be borne in mind that the damping 
of the receiver circuit has also a large influence on the form of the 
resonance curve, and that good isolation can not be obtained unless 
the receiving circuit also has a small decrement. Under favorable 
conditions we can employ a sending key, which does not interrupt 
the production of the electric waves at the sending station, but sim- 
ply alters the wave length slightly by about one-fourth per cent. If, 
then, the corresponding receiving station has a feebly damped re- 
ceiver, this change will be sufficient to cut up the continuous record 
or telephone sound at that station into Morse dots and dashes, and so 
transmit signals. But another station not so tuned will either re- 
ceive nothing at all or else a continuous unbroken line or sound not 
having any meaning. There are other methods by which signals not 
intended for a particular receiver can be rejected by it. Fessenden 
has described for this purpose an interference detector, in which the 
impulses it is not desired to receive are made to divide between two 
paths, the oscillations in which are then caused. to neutralize each 
other’s effect on the oscillation detector. On the other hand, the 
waves of the wave length it is desired to receive do not so neutralize 
themselves, but produce a signal by their operation on the detector. 
We must pass on to notice in the next place some improvements in 
oscillation detectors, and means of testing them. As already ex- 
plained, the ether waves sent out by the transmitting antenna fall on 
the receiving antenna and create in it or some other circuit connected 
to it very feeble oscillations. These oscillations being very feeble 
alternating currents of high frequency, can not directly affect either 
an ordinary telegraphic instrument or a telephone, but we have to 
interpose a device of some kind called an oscillation detector, which 
is affected by oscillations in such a manner that it undergoes some 
