RADIOTELEGRAPH Y—FLEMING.,. ie 
The space occupied by such closed antenne has hitherto prevented 
their employment on ships. ‘There is still, therefore, an opening for 
the invention of apparatus capable of being used on board ship 
which will enable one ship to locate, within narrow limits, the 
direction of another ship sending signals to it, and therefore of 
ascertaining immediately the direction from which some call for help 
is proceeding. 
Closely connected with this part of the subject is the question so 
frequently discussed as to the isolation or secrecy of radiotelegraphic 
communication. Up to the present moment the only really practical 
method of isolating any particular receiver so as to make it sensitive 
only to signals coming from a certain direction, is to avail ourselves 
to the utmost of the principle of resonance and to tune the sending 
and receiving circuits to exact cor- Th 
respondence. The question then 
arises, What is it which determines 90 
the effectiveness of this tuning? fe 
If waves of one particular wave 
jength are impinging on a receiving : “0 
antenna and creating signals, by % ,, 
how much can the wave length be 
varied or the tuning of the receiver é 7 
upset or changed without prevent- gs 40 
ing these signals being received? § 
It is clear that the narrower this § ¢ 
range the more perfect the isola- 20 
tion of the receiver. It can be 
shown that it depends upon the 
form of the resonance curve of the ; pees 
sending and _ receiving circuits. Percentage Variation from Exact Tuning, 
If the sending station is emitting Fic. 14.—Resonance curves. 
waves of a certain constant wave length and damping or decrement, 
then in the receiving circuit of all other stations within range there 
will be produced oscillations having a certain mean square value 
measurable by appropriate instruments. If any receiving circuit is 
gradually brought by adjustment of its capacity and inductance into 
exact syntony or tune with the sending station, then this receiver cur- 
rent reaches its maximum value and there is a definite lesser value of 
the receiver current for every particular degree of want of tuning or 
dissonance between the two. The curve which by its ordinates ex- 
presses this receiver current corresponding to each particular tuning 
or natural frequency of the receiving circuit, is called a resonance 
curve (see fig. 14). If this curve has a very sharp peak, then it 
clearly indicates that a slight want of tuning or syntony between the 
stations will greatly reduce the receiver current. The peakiness of 
