MEANS FOR PREVENTING INDUCTION IN TELEPHONIC CIRCUITS
Year: 1887

page_text: I, STEPHEN CURTIS DREW, of Boston, in the County of Suffolk, Commonwealth of
Massachusetts, and United States of America, Clerk, do hereby declare the nature of
my invention for "MEANS FOR PREVENTING INDUCTION IN TELEPHONIC CIRCUITS," and
in what manner the same is to be performed, to be particularly described and
ascertained in and by the following statement :-
The object of my invention is to suppress induction in electrical conductors, and
I accomplish this object by interposing at any point where it is desired the in-
duction should be cut off an obstacle to the passage of the induced current, which,
however, will have no effect upon the battery current. It is intended to be applied
to telephones, and when so applied my device is to be placed near the transmitter.
The way in which I purpose to carry my invention into practice is to employ
two coils; one as a primary induction coil, and the other as a secondary induction
coil. These coils may be of the ordinary kind, consisting of insulated copper wire
wound upon spools, and, preferably, provided with soft iron cores, although other
cores may be used. One of these coils is put in circuit with the wire leading to
the battery, so that the battery current will pass through it on its way to the
transmitter. Close beside this first mentioned coil, or end to end with it, is placed
a second coil, which is connected with the telephone. The vibrations passing over
the wire leading from the transmitter will pass by induction to the second induction
coil, while the induced currents which may have effected the conductor will com-
pletely cut off all intervening space between the two coils, and will not affect at
all the coil which is connected with the telephone. The coils are to be adjusted
with reference to their distance from each other so as to have them as near together
as possible and yet sufficiently far apart to stop induced currents. Ordinarily
they should be placed as near to each other as possible, without touching, so that the
rims of the spools nearly touch when they are placed side by side; and when they
are placed end to end, the ends of the spools and metal coils should nearly touch.
Instead of two coils forming together an induction coil, as above described, the
ordinary form of the induction coil may be used, consisting of a primary coil of
comparatively few turns of wire, a central iron core and secondary coil outside the
primary coil, having a greater number of turns of wire.
I place my coils in such relation to one another that the currents of electricity

page_text: act on the receiver by direct induction from the primary coil to the secondary.
Where I use two coils placed side by side, or end to end, it is essential, of course,
that the cores of these coils should be entirely separated one from the other; and
it is also essential that the coils should be so near each other that the primary
will induce directly on the secondary coil. Where I use the coils in the ordinary
form of an induction coil, that is, the primary within the secondary, the primary
coil and its core together induce upon the secondary coil, so that the induction of
the secondary coil is a combination of magnetic and electric induction.
It will be seen that my invention is based upon the discovrey which I have made;
that, while the current from the transmitter is of such strength as to pass by induction to
the coil connected with the telephone so as not to affect the tone, yet the induced
currents are so weak as not to be able to so pass, and the result is that the induction
is completely shut off and nothing can be heard in the telephone but the sounds
transmitted by the current from the transmitter over the conductor connected with
it the current from the transmitter passing directly through the primary coil by
induction to the secondary coil, as already stated. I design using my invention
either with the ordinary earth circuit or the metallic circuit; and it may be applied
to any system of electric telephony, but in practice I prefer to use it in connection
with the system now generally in use, in which a transmitter is used, known as
the "Blake transmitter," in which are employed the primary of an induction coil, a
local circuit with a microphone and battery, and secondary coil connected with the
main line.
In the drawings I have shown my invention as applied to a telephone and
transmitter, the transmitter being of the kind commonly known as the "Blake
transmitter." Fig. 1, shows the coils separate from each other. Fig. 2, shows the
primary coil within the secondary, in the ordinary way. a is an electrical conductor
over which the current from the transmitter passes; b is a primary induction coil;
c is a secondary coil; d is a telephone; e is a transmitter; is the conductor leading
to the ground, and 9 is a screw by means of which the two coils may be adjusted
with reference to each other.
Having now particularly described and ascertained the nature of my inven-
tion, and in what manner the same is to be performed, I declare that what I
claim is :-
1. In a telephonic circuit, the combination of a telephone having primary and
secondary circuits, of the induction coil b, c, the coil b, being placed in the above
secondary circuit, substantially as and for the purpose set forth.
2. In a telephonic circuit, the combination of a telephone having primary and
secondary circuits, of the induction coil b, c, the coil b, being placed in the above
secondary circuit, and the secondary circuit being made adjustable so that it can be
placed at a greater or lesser distance from the primary coil, substantially as and for
the purpose described.
3. A line and apparatus for telephonic communication comprising a main line
grounded at either end and including in its circuit at each station the primary coil
of one induction coil, and the secondary coil of another induction coil, combined with
a receiving telephone and short local circuit therefor, including the secondary coil of
the first mentioned induction coil, and a local circuit including a battery microphone
and primary coil of the other induction coil, the whole combined and arranged
substantially as and for the purpose specified.
Datad this 20th day of October 1887