A pair of high resistance telephone receivers in nice adjustment
constitute one of the most sensitive electrical instruments in existence
and will detect an exceedingly weak current.
The only type of receiver of much service in wireless telegraphy is that
known as a watch case or pony receiver. It is small and compact so that
it may be attached to a headband and clamped against the ear.
[Illustration: Fig. 142. Types of Permanent Magnets.]
The permanent magnets of a watch-case receiver are usually in the form
of either a ring or a horseshoe as shown by Fig. 142. The first form has
"consequent" poles and is considered somewhat superior to the horseshoe,
since the lines of force are not so liable to pass across the pole
pieces before they pass through the electromagnets and the diaphragm.
The ordinary low resistance telephone receiver is perfectly well suited
to the telephone work for which it was designed and adjusted, and will
give good service on a wireless receptor for short distances, but can be
considerably improved by following the suggestions and instructions
given below.
The principal objection to the ordinary receiver is that it does not
contain enough turns of wire on its bobbins. This is easily remedied by
carefully rewinding them with a very fine silk covered, pure copper
magnet wire no larger than No. 40 B. S. gauge. This will increase the
number of turns and also the resistance, but it must not be inferred
that resistance is to be desired. This is a common impression of
amateurs who do not understand the underlying principle, that the
strength of an electromagnet varies directly as the number of turns of
wire, multiplied by the amperes flowing through the magnet. When a
telephone receiver is wound with a finer wire the resistance is
increased, cutting down both the current and the strength of the magnet.
But if pure copper wire is used, and the winding not carried beyond the
point where the circumference of the outside layer becomes twice as
great as the circumference of the first layer, the number of turns
increases faster than the resistance and the magnet strength is
considerably greater than before the receiver was rewound.
No. 40 B. S. gauge silk covered wire is often used for this purpose, but
the best results are obtained with enameled covered wire of the same
size. It is possible to wind almost three times as much of the enameled
wire on a telephone bobbin as silk wire of the same size. The difference
is due to the thickness of the insulation. An ordinary double pole
watch-case receiver will have a resistance of 800-1,000 ohms when wound
with silk covered wire and 1,500-1,800 ohms when wound with enameled
wire.
To rewind a telephone receiver, first unscrew the cap and remove the
diaphragm, then remove the bobbins by unloosening the screws with the
aid of a screw driver. Unwind the old wire and examine the empty bobbins
to see that wherever the wire is liable to come into contact with the
metal that it is well insulated with paraffined paper or some other
equally good material. Then wind the new wire on in smooth even layers
and when it is completed fasten the bobbins back on the permanent
magnets and connect them up. The current should flow through in opposite
directions so that the north pole of one and the south pole of the other
is on top. Do not trust splice connections but solder them using acid as
a flux.
The Navy Department specifies that its wireless receivers shall be wound
with copper wire of not less than 0.0015 inch in diameter and the
diaphragm to have a diameter of 1 3/4 inches and a thickness of 0.004
inch. The resistance of the coils is specified at 1,000-1,100 ohms.
There is not much advantage in greatly exceeding the number of turns
possible with this winding, for to obtain them a much finer wire than
No. 40 B. S. gauge is necessary and the ratio between resistance and
turns becomes greater.
The second objection to the ordinary receiver is that the diaphragms are
very often too thick. A receiver having a thin diaphragm is preferable
because when a weak current is sent through the coils, the change in
magnet strength is greater. But this may be carried to excess and the
diaphragm made so thin that it cannot absorb sufficient lines of force
to properly play its part. The best thickness then for a diaphragm can
only be determined experimentally and depends much upon the diameter.
The distance from the poles and the strength of the magnets will also
have considerable bearing on the thickness. The ordinary phone will be
very sensitive and give clear tones with diaphragms ranging from
.01-.004 inch.
The relation between the thickness and the diameter is shown by the
following: If the diaphragm of a receiver is increased in diameter, the
tones will become more distinct, but if the increase is carried too far
they will become indistinct and the only remedy is to thicken the
diaphragm. Likewise if after clearness is secured the diaphragm is
thickened so that the tones again become indistinct, the diameter must
be increased.
[Illustration: Fig. 143. Grinding Tool.]
The third objection is that such receivers are not carefully and
properly adjusted. The adjustment is also a matter of experiment and is
accomplished by comparison of the receiver in question with one which is
known to be in a sensitive condition. The adjusting may be done by means
of the tool shown in Fig. 143.
This tool is made from an ordinary file by grinding off the teeth on one
side save for a distance of about 3/4 inch in the middle. The grinding
may be done on an emery wheel. The part (_a_) is used for filing the
pole pieces and thus making the distance between them and the diaphragm
greater. The tool has the advantage over an ordinary file of permitting
the poles to be filed without removal from the receiver and without
grooving the diaphragm bed or the receiver case.
To lessen the distance between the poles and the diaphragm, lay the
receiver bed downward on a piece of fine emery paper and rub with a
circular scouring motion. If the emery paper is placed on a perfectly
flat surface no trouble will be experienced in grinding the bed down
evenly.
When filing the pole pieces rub with the same circular motion so as to
grind off all sides evenly. Test from time to time by passing a straight
edge over the bed in all directions while holding to the light and
looking between the straight edge and the poles. In this manner the
distance separating the diaphragm and the poles may be gauged and
whether or not it is the same on all sides. Bear in mind that if the
diaphragm is thin, the attraction of the permanent magnets will cause it
to bend in towards the poles.
In case you have a pair of receivers built for wireless work, which
appear to be in good condition but do not give their former results, the
last thing to do is to tamper with the adjustment. The most common
cause, when the tones are impaired, is dirt or dust accumulated on the
poles or diaphragm and damping its vibration. The cap should be
carefully unscrewed and the diaphragm examined to see if it is bent. If
so, replace with a new one of the same size. Remove any dirt or filings,
and if the diaphragm is rusty clean it by laying it on a flat surface
and rubbing it with a piece of fine emery paper. Then give it a thin
coat of colorless lacquer. Examine the magnets and pole pieces to see if
they have become loosened and if so tighten them.
Or the trouble may be that the permanent magnets have lost part of their
magnetism, and almost any receiver which has been in use for some length
of time will bear having its magnets strengthened. If they are found to
be weak they should be removed and remagnetized. This is accomplished by
winding a coil of No. 18 B. S. gauge wire around them and sending a
heavy direct current through for a few minutes.
[Illustration: Fig. 144. Parts of a Holtzer Cabot Receiver.]
In carrying out any of these suggestions remember to work with one
receiver at a time, keeping the other for comparison, so that by
repeated tests you may tell whether or not an improvement is being made
and when well enough is reached, let it alone.
Fig. 144 shows the construction of a telephone receiver manufactured by
the Holtzer Cabot Co. of Brookline, Mass. The permanent magnets and
bobbins are mounted in a metal cup, _B_, which supports the diaphragm,
_A_. The metal cup is enclosed in a hard rubber shell, _C_, and fitted
with a cap, _D_.
The complete receivers are mounted on an adjustable headband and fitted
with pneumatic ear cushions which make them set more comfortably and
shut out extraneous noise. These receivers are wound to all resistances
used in the wireless field but, for the experimenter, those having a
resistance of 1000 ohms apiece will give the best all around results.
[Illustration: Fig. 145. Holtzer Cabot Head Set.]
It is very desirable that the receivers should be fitted with a
comfortable headband which will hold the telephones in close adjustment
to the ears. Fig. 146 shows the construction of such a band.
[Illustration: Fig. 146. Adjustable Head Band.]
Two brass straps, 1 inch wide, 12 inches long and 1/16 inch thick are
necessary. Two 1/8-inch holes, _mm_, are bored in them 3/16 inch from
one end and the end bent up at an angle along a line 3/8 inch from the
end as shown by the drawing. A cover is made for the straps, by sewing
two strips of leather 1 1/4 inches wide and 12 inches long, together
along their edges. The covers may then be slipped over the straps. Two
1/16-inch brass strips, 3/8 inch wide and a little longer than one-half
the circumference of the watch-case receiver to be fitted to the head
band, are bent into stirrups as shown in the illustration by _Y_. The
ends of the stirrups are fitted with two pivots, _PP_, which are riveted
in a hole. Two holes or recesses are bored in the shell of each
receiver, on the sides 180 degrees apart. The pivots, _PP_, fit into
these holes and form bearings in which the receivers can turn and adjust
themselves to the ears. The stirrup has two small holes, _hh_, bored 3/8
inch apart at the top and tapped to receive a small screw. The screws
pass through the holes, _mm_, and hold the stirrups at the end of their
respective strap.
The leather-covered straps are bound together by means of two clamps
which permit the head band to be adjusted to suit the wearer. The
clamps, _C_, are made from 1/16-inch brass 3/8 inch wide. Their form and
construction may be best understood from the drawing. A small screw eye
is cut in half and soldered to the center of the upper side of the clamp
so that the receiver cord may be passed through the eyelet, _E_.
The telephone receivers should be connected in series by means of a
flexible telephone cord.