IMPROVEMENTS IN TELEPHONIC CIRCUITS
Year: 1885

page_text: COMPLETE SPECIFICATION.
I SILVANUS PHILLIPS THOMPSON of University College Bristol in the County
and City of Bristol Doctor of Science and Professor of Experimental Physics do
hereby declare the nature of this invention for "IMPROVEMENTS 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:-
My Invention has for its object to neutralize the self inductive effects and
tendency to sparking, in direct working telephonic systems, arising from the
magnetism of the electro magnetic coil in the receiver, and from the self induction
of the line wire, and this I effect by compensating the circuits as hereinafter
explained with reference to the accompanying drawings.
In figure 1 the Electro Magnet in the Receiver R constitutes with the line L one
branch of the balanced circuit and the electro magnet wound upon the Core C C¹
constitutes the other branch-The electro-magnet should preferably be so wound
that the ratio of its co-efficient of self induction to its resistance should be at least
as great as, or greater than, the ratio of the co-efficient of self induction of the line
and of the coils of the receiver taken together, to the total resistance they offer.
The transmitter and battery in Figure 1. are outside the balanced circuit, the
current from the battery passing through the Transmitter to A where the balanced
circuit divides-In general it is preferred by me that the coil of the balancing
Electro magnet should be wound of many turns of thick wire so as to have a small
resistance and yet a high co-efficient of self induction-There should also be plenty
of iron in the core which should be a long one, and for this purpose need not be
made up of bundles of thin wires.
Figures 2, 3 and 4 illustrate further methods of arranging the compensated
circuits the balancing electro magnet wound around the core C O¹ being in Figures
2 and 3 considered as part of the divided circuit-In figure 4 although the current
divides at A and B, part going through the line and receiver and part going
through the Transmitter, it is convenient to regard the Electro Magnet wound on
the core C C¹ as the part of the circuit which is balanced against the self induction
of the line and receiver, for if the circuit were opened at T the tendency of the
Electro magnet wound on C C¹ to send a spark through T will be counteracted
and neutralized by the self induction of the line and receiver, and vice versa on
closing T when the tendency of the self induction of the line and receiver to send
back a spark through T will be counteracted and balanced by the self induction

page_text: of the coil wound upon C C¹. For practical purposes the arrangements of Figures
2 and 4 are identical-Figure 3 shews a somewhat similar arrangement of circuits
in which two transmitters are introduced. In figure 5 the battery line and
receiver are included in one branch of the circuit and the electro magnet C C¹
balances their tendency to send a spark through the Transmitter T, for the closing
of the circuit at T diminishes the magnetism in the electro-magnet C C¹ and
increases the magnetism in the Electro-magnet in the distant receiver, and the
opening of the circuit at T increases the magnetism in the electro-magnet C C¹,
and decreases the magnetism of the electro-magnet in the distant receiver, and
these effects in these two branches of the telephonic circuit tend to balance one
another thus diminishing the tendency to spark at T.
Having now particularly described and ascertained the nature of my said
Invention and in what manner the same is to be performed I declare that what
I claim is
In a direct working telephonic system the method of compensating the circuits
substantially as hereinbefore described by the use of an electro magnetic coil
wound upon an iron Core to neutralize the self inductive effects and tendency to
spark arising from the magnetism of the electro magnetic coil in the receiver and
from the self induction of the line wire substantially as set forth.
Dated this 29th day of May 1885.
J. HENRY JOHNSON,
47, Lincoln's Inn Fields, London, W.C.,
Agent for the said Silvanus Phillips Thompson.