An Improved Board for Mounting Telephonic Switches and other Electrical Apparatus.
Year: 1886

page_text: Date of Application, 30th Dec 1886
Complete Left, 29th Sept., 1887
Complete Accepted, 1st Nov, 1887
An Improved Board for Mounting Telephonic Switches and other
Electrical Apparatus.
I, ALFRED ROSLING BENNETT of 90 George Street, Edinburgh, Telegraph Engineer,
do hereby declare the nature of this invention to be as follows:-
In mounting the switching apparatus employed in telephonic exchanges, the
indicators, spring jacks, terminals, lightning-guards, and other appurtenances, are
necessarily for economy of space and convenience of operating, fixed in very close
proximity. Unless the wood of which the board, frame or table, to which the
switching appliances are fixed, is composed of thoroughly dried or seasoned wood,
and the board, frame or table after completion is kept constantly in a very dry
atmosphere, a condition difficult to attain in practice, the electrical currents leak
from one appliance to another by means of the moisture in the pores of the wood.
This leakage affects the efficiency of the telephone exchange materially. In ringing
the subscribers bells and in calling the attention of the operator, alternating currents
of considerable tension are frequently used and these currents leak through the damp
wood to appliances belonging to subscribers' lines for which they are not intended and
cause clicking sounds in the telephones, connected to those lines, which clicking sounds
detract from the clearness of the speech passing through those telephones. The wood used
in some telephone exchanges is so damp that speech itself intended for one subscriber
leaks across to the lines of other subscribers for whom it is not intended and becomes
audible in their telephones. It is consequently of importance in a telephone exchange to
prevent this cross leakage without employing such expensive material for the boards,
frames andtables as ebonite. To this end, I take the wood of which a switch board, frame
or table is to be made and dry it thoroughly, preferably in an oven or kiln. I prefer to
make all the screw holes and other openings used in fixing the appliances before drying
the wood. The wood being thoroughly dried, I immerse it in a liquid incapable of
conducting electricity and which does not mix readily with water, and allow it to soak
until it becomes thoroughly saturated therewith. The immersion should take place
while the wood is hot from the oven.

page_text: The non-conducting liquid may be a solid liquefied by heat, such as animal fat or
paraffin wax or a chemical solution which has the property of becoming solid after
absorption by the pores of the wood, such as india-rubber or other vegetable gum in
one of its solvents, or silicate of potash or soda.
The wood treated by this process is found to have become incapable of conducting
any electrical currents, sufficiently strong to affect a telephone, from one appliance to
another, however closely together the appliances may be fixed. The wood will also
have become incapable of absorbing moisture and has consequently become permanently
insulating.
The insulating substance used must have three essential qualities. Firstly, it must
be a good electrical insulator; secondly it must have the power of resisting moisture;
thirdly, it must be of a permanent nature, that is to say, not liable to evaporation or
chemical change after having been taken into the wood, unless such evaporation
or chemical change be unattended by loss of either or both of the two first-named
qualities.
For the purpose of insulating the wood used for telephonic switching apparatus, I
find boiled linseed oil, melted paraffin wax and melted sulphur very efficient, but many
other substances may be substituted.
Although for ordinary purposes simple immersion of the wood for several hours in
the insulating liquid will suffice, a higher degree of efficiency may be attained by
forcing the liquid into the wood by means of a pump or piston working in a cylinder
as is well understood in mechanics.
Although my invention is intended chiefly to improve the insulation of telephonic
switching appliances it may advantageously be used in connection with the mounting
of other electrical apparatus.
Dated this Twenty ninth day of December 1886.
CRUIKSHANK & FAIRWEATHER,
62, Saint Vincent Street, Glasgow,
Agents.

page_text: COMPLETE SPECIFICATION.
An Improved Board for Mounting Telephonic Switches and other
Electrical Apparatus.
I, ALFRED ROSLING BENNETT of 904 George Street Edinburgh Telegraph Engineer
do hereby declare the nature of this invention and in what manner the same is to be
performed, to be particularly described and ascertained in and by the following
statement:-
In mounting, the switching apparatus employed in telephonic exchanges the
indicators, spring jacks, terminals lightning-guards, and other appurtenances, are
necessarily for economy of space and convenience of operating, fixed in very close
proximity. Unless the wood of which the board frame or table to which the switching
appliances are fixed, is composed of thoroughly dried or seasoned wood, and the
board, frame or table after completion is kept constantly in a very dry atmosphere, a
condition difficult to attain in practice, the electric currents leak from one appliance
to another by means of the moisture in the pores of the wood. This leakage affects
the efficiency of the telephone exchange materially. In ringing the subscribers' bells
and in calling the attention of the operator, alternating currents of considerable
tension are frequently used and these currents leak through the damp wood to
appliances belonging to subscribers' lines for which they are not intended and cause
clicking sounds in the telephones, connected to those lines, which clicking sounds
detract from the clearness of the speech passing through those telephones. The wood
used in some telephone exchanges is so damp that speech itself intended for one
subscriber leaks across to the lines of other subscribers for whom it is not intended
and becomes audible in their telephones. It is consequently of importance in a
telephone exchange to prevent this cross leakage without employing such expensive
material for the boards, frames and tables as ebonite. To this end, I take the wood
of which a switch-board, frame or table is to be made and dry it thoroughly, preferably
in an oven or kiln. I prefer to make all the screw holes and other openings used in
fixing the appliances before drying the wood. The wood being thoroughly dried I
immerse it in a liquid incapable of conducting electricity and which does not mix readily
with water, and allow it to soak until it becomes thoroughly saturated therewith. The
immersion should take place while the wood is hot from the oven.
The non-conducting liquid may be a solid liquefied by heat, such as animal fat
or paraffin wax; or a chemical solution which has the property of becoming solid
after absorption by the pores of the wood, such as india-rubber or other vegetable gum
in one of its solvents or silicate of potash or soda. The wood treated by this process
is found to have become incapable of conducting any electrical currents, sufficiently
strong to affect a telephone, from one appliance to another, however closely together
the appliances may be fixed. The wood will also have become incapable of absorbing
moisture and has consequently become permanently insulating.

page_text: The insulating substance used must have three essential qualities. Firstly, it must
be a good electrical insulator; secondly it must have the power of resisting moisture;
thirdly, it must be of a permanent nature, that is to say not liable to evaporation, or
chemical change after having been taken into the wood unloss such evaporation or
chemical change be unattended by loss of either or both of the two first-named
qualities.
For the purpose of insulating the wood used for telephonic switching apparatus, I
find linseed oil raw or boiled, specially effective. When raw oil is used, the wood
soaked with it should afterwards be baked or otherwise subjected to heat, until the oil
in the pores has becomes oxidised. Other insulating agents which are effective are
shellac, resin, and other gums; animal and vegetable fats; paraffin wax; sulphur;
silicates; soluble glass; india-rubber and gutta-percha, applied in a state of solution
or fusion. Non-volatile oils may be employed and such of the insulating oxides and
other salts which are soluble in non-conducting solvents or are capable of being
liquefied by fusion.
Although my invention is intended chiefly to improve the insulation of telephonic
switching appliances it may advantageously be used in connection with the mounting
of other electrical apparatus.
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 :—
As a mount for telephonic switches and other electrical apparatus a board composed
of wood impregnated or treated with insulating material substantially as and for the
purpose set forth.
Dated this Twenty eighth day of September 1887