ON ELECTRIC BELLS AND OTHER SIGNALLING APPLIANCES.
[S] 41. An electric bell is an arrangement of a cylindrical soft iron
core, or cores, surrounded by coils of insulated copper wire. On causing
a current of electricity to flow round these coils, the iron becomes,
_for the time being_, powerfully magnetic (see [S] 13). A piece of soft
iron (known as the _armature_), supported by a spring, faces the magnet
thus produced. This armature carries at its free extremity a rod with a
bob, clapper or hammer, which strikes a bell, or gong, when the
armature, under the influence of the pull of the magnet, is drawn
towards it. In connection with the armature and clapper is a device
whereby the flow of the current can be rapidly interrupted, so that on
the cessation of the current the iron may lose its magnetism, and allow
the spring to withdraw the clapper from against the bell. This device is
known as the "contact breaker" and varies somewhat in design, according
to whether the bell belongs to the _trembling_, the _single stroke_, or
the _continuous ringing_ class.
[S] 42. In order that the electric bell-fitter may have an intelligent
conception of his work, he should _make_ a small electric bell himself.
By so doing, he will gain more practical knowledge of what are the
requisites of a good bell, and where defects may be expected in any he
may be called upon to purchase or examine, than he can obtain from pages
of written description. For this reason I reproduce here (with some
trifling additions and modifications) Mr. G. Edwinson's directions for
making an electric bell:--[10]
_How to make a bell._--The old method of doing this was to take a piece
of round iron, bend it into the form of a horse-shoe, anneal it (by
leaving it for several hours in a bright fire, and allowing it to cool
gradually as the fire goes out), wind on the wire, and fix it as a
magnet on a stout board of beech or mahogany; a bell was then screwed to
another part of the board, a piece of brass holding the hammer and
spring being fastened to another part. Many bells made upon this plan
are still offered for sale and exchange, but their performance is always
liable to variation and obstruction, from the following causes:--To
insure a steady, uniform vibratory stroke on the bell, its hammer must
be nicely adjusted to move within a strictly defined and limited space;
the least fractional departure from this adjustment results in an
unsatisfactory performance of the hammer, and often a total failure of
the magnet to move it. In bells constructed on the old plan, the wooden
base is liable to expansion and contraction, varying with the change of
weather and the humidity, temperature, etc., of the room in which the
bells are placed. Thus a damp, foggy night may cause the wood to swell
and place the hammer out of range of the bell, while a dry, hot day may
alter the adjustment in the opposite direction. Such failures as these,
from the above causes alone, have often brought electric bells into
disrepute. Best made bells are, therefore, now made with metallic
(practically inexpansible) bases, and it is this kind I recommend to my
readers.
[Footnote 10: "Amateur Work."]
[Illustration: Fig. 17.]
[Illustration: Fig. 18.]
_The Base_, to which all the other parts are fastened, is made of 3/4
in. mahogany or teak, 6 in. by 4 in., shaped as shown at Fig. 17, with a
smooth surface and French polished. To this is attached the metallic
base-plate, which may be cut out of sheet-iron, or sheet-brass (this
latter is better, as iron disturbs the action of the magnet somewhat),
and shaped as shown in Fig. 18; or it may be made of cast-iron, or cast
in brass; or a substitute for it may be made in wrought-iron, or brass,
as shown in Fig. 19. I present these various forms to suit the varied
handicrafts of my readers; for instance, a worker in sheet metal may
find it more convenient to manufacture his bell out of the parts
sketched in Figs. 17, 18, 20^A, 21, 23, 24^A, and 25; but, on the other
hand, a smith or engineer might prefer the improved form shown at Fig.
31, and select the parts shown at Figs. 20^A, 22, 19, choosing either to
forge the horse-shoe magnet, Fig. 20, or to turn up the two cores, as
shown at Fig. 21 (A), to screw into the metal base, Fig. 21 B, or to be
fastened by nuts, as shown at Fig. 19. The result will be the same in
the end, if good workmanship is employed, and the proper care taken in
fixing and adjusting the parts. A tin-plate worker may even cut his
base-plate out of stout block tin, and get as good results as if the
bell were made by an engineer. In some makes, the base-plate is cut or
stamped out of thick sheet-iron, in the form shown by the dotted lines
on Fig. 18, and when thus made, the part A is turned up at right angles
to form a bracket for the magnet cores, the opposite projection is cut
off, and a turned brass pillar is inserted at B to hold the contact
screw, or contact breaker ([S] 41).
[Illustration: Fig. 19.]
[Illustration: Fig. 20.]
[Illustration: Fig. 20 A.]
The _Magnet_ may be formed as shown at Fig. 20, or at Fig. 20^A. Its
essential parts are: 1st. Two soft iron cores (in some forms a single
core is now employed); 2nd. An iron base, or yoke, to hold the cores
together; 3rd. Two bobbins wound with wire. The old form of magnet is
shown at Fig. 20. In this form the cores and yoke are made out of one
piece of metal. A length of round Swedish iron is bent round in the
shape of a horseshoe; this is rendered thoroughly soft by annealing, as
explained further on. It is absolutely essential that the iron be very
soft and well annealed, otherwise the iron cores retain a considerable
amount of magnetism when the current is not passing, which makes the
bell sluggish in action, and necessitates a higher battery power to make
it work (see [S] 14). Two bobbins of insulated wire are fitted on the
cores, and the magnet is held in its place by a transverse strip of
brass or iron secured by a wood screw passing between the two bobbins.
The size of the iron, the wire, the bobbins, and the method of winding
is the same as in the form next described, the only difference being
that the length of the iron core, before bending to the horse-shoe
form, must be such as to allow of the two straight portions of the legs
to be 2 in. in length, and stand 1-3/8 apart when bent. We may now
consider the construction of a magnet of the form shown at Fig. 20^A. To
make the cores of such a magnet, to ring a 2-1/2 in. bell, get two 2
inch lengths of 5/16 in. best Swedish round iron, straighten them,
smooth them in a lathe, and reduce 1/4 in. of one end of each to 4/16 of
an in., leaving a sharp shoulder, as shown at Fig. 21 A. Next, get a
2-in. length of angle iron, drill in it two holes 1-3/8 apart, of the
exact diameter of the turned ends of the cores, and rivet these securely
in their places; this may be done by fastening the cores or legs in a
vice whilst they are being rivetted. Two holes should be also bored in
the other flange to receive the two screws, which are to hold the magnet
to the base, as shown at Fig. 21 B. The magnet is now quite equal to the
horse-shoe form, and must be made quite soft by annealing. This is done
by heating it in a clear coal fire to a bright red heat, then burying it
in hot ashes, and allowing it to cool gradually for a period of from 12
to 24 hours; or perhaps a better guide to the process will be to say,
bury the iron in the hot ashes and leave it there until both it and they
are quite cold. The iron must be brought to a bright cherry red heat
before allowing it to cool, to soften it properly, and on no account
must the cooling be hurried, or the metal will be _hard_. Iron is
rendered hard by hammering, by being rapidly cooled, either in cold air
or water, and hard iron retains magnetism for a longer time than soft
iron. As we wish to have a magnet that will only act as such when a
current of electricity is passing around it, and shall return to the
state of a simple piece of unmagnetised iron when the current is broken,
we take the precaution of having it of soft iron. Many bells have failed
to act properly, because this precaution has been neglected, the
"residual" (or remaining) magnetism holding down the armature after
contact has been broken. When the magnet has been annealed, its legs
should be polished with a piece of emery cloth, and the ends filed up
level and smooth. If it is intended to fasten the cores into the
base-plate, this also should be annealed, unless it be made of brass, in
which case a thin strip of soft iron should connect the back ends of the
two legs before they are attached to the brass base (an iron yoke is
preferable, as it certainly is conducive to better effects to have a
massive iron yoke, than to have a mere strip as the connecting piece).
It will also be readily understood and conceded that the cores should be
cut longer when they are to be fastened by nuts, to allow a sufficient
length for screwing the ends to receive the nuts. The length and size of
the legs given above are suitable for a 2-1/2 in. bell only; for larger
bells the size increases 1/16 of an inch, and the length 1/4 of an inch,
for every 1/2 in. increase in the diameter of the bell.
[Illustration: Fig. 21.]
The _Bobbins_, on which the wire that serves to carry the magnetising
current is to be wound, next demand our attention. They may be turned
out of boxwood, ebony, or ebonite, or out of any hard wood strong enough
and dense enough to allow of being turned down thin in the body, a very
necessary requirement to bring the convolutions of wire as near the coil
as possible without touching it. Some amateurs use the turned ends of
cotton reels or spools, and glue them on to a tube of paper formed on
the cores themselves. If this tube be afterwards well covered with
melted paraffin wax, the plan answers admirably, but of course the
bobbins become fixtures on the magnets. There are some persons who are
clever enough to make firm bobbins out of brown paper (like rocket
cases), with reel ends, that can be slipped off and on the magnet cores.
To these I would say, "by all means at your command, do so if you can."
The size of the bobbins for a 2-1/2 in. bell should be: length 1-3/4
in., diameter of heads 3/4 of an in., the length increasing 1/4 of an
in. and the diameter 1/8 of an in. for every additional 1/2 in. in the
diameter of the bell. The holes throughout the bobbins should be of a
size to fit the iron cores exactly, and the cores should project 1/8 of
an inch above the end of the bobbins when these are fitted on. The wire
to be wound on the bobbins is sold by all dealers in electrical
apparatus. It is copper wire, covered with cotton or with silk, to
ensure insulation. Mention has already been made of what is meant by
insulation at [S] 3, but, in order to refresh the reader's memory, Mr. G.
Edwinson's words are quoted here. "To insulate, as understood by
electricians, means to protect from leakage of the electric current, by
interposing a bad conductor of electricity between two good conductors,
thus insulating[11] or detaching them from electric contact."
[Footnote 11: _Insula_ in Latin means an island, hence an electrified
body is said to be insulated when surrounded by non-conductors, as an
island by the sea.]
The following list will enable my readers to see at a glance the value
of the substances mentioned here as conductors or insulators, the best
conductors being arranged from the top downwards, and the bad conductors
or insulators opposed to them in similar order, viz., the worst
conductors or best insulators being at the top:--
_Conductors._ _Insulators._
Silver. Paraffin Wax.
Copper. Guttapercha.
Iron. Indiarubber.
Brass. Shellac.
All Other Metals. Varnishes.
Metallic Solutions. Sealing Wax.
Metallic Salts. Silk and Cotton.
Wet Stone. Dry Clothing.
Wet Wood. Dry Wood.
Oil, Dirt and Rust.
See also the more extended list given at [S] 5 for a more complete and
exact classification.
It will be seen, on reference to the above, that copper is a good
conductor, being excelled by silver alone in this respect; and that silk
and cotton are bad conductors. When, therefore, a copper wire is bound
round with silk or with cotton, even if two or more strands of such a
covered wire be superimposed, since these are electrically separated by
the non-conducting covering, no escape of electricity from one strand to
the other can take place, and the strands are said to be insulated. If
the copper wire had been coiled _naked_ round a bobbin, each convolution
touching its neighbour, the current would not have circled round the
whole length of the coils of wire, but would have leapt across from one
coil to the other, and thus the desired effect would not have been
obtained. A similar result, differing only in degree, would occur if a
badly insulating wire were used, say one in which the covering had been
worn in places, or had been badly wound, so as to expose patches of bare
copper wire. If the insulation of a wire be suspected, it should be
immersed in hot melted paraffin wax, and then hung up to drain and cool.
The size of wire to be used on a 2-1/2 in. bell should be No. 24 B. W.
G., the size falling two numbers for each 1/2 in. increase in the
diameter of the bell. In these wires the higher the number, the finer
the size, No. 6 being 1/5 and No. 40 being 1/200 of an inch in diameter.
Silk-covered wire has an advantage over cotton-covered wire, inasmuch as
the insulating material occupies less space, hence the convolutions of
wire lie closer together. This is important, as the current has less
effect on the iron if removed further from it, the decrease being as
the _square_ of the distance that the current is removed from the wire.
Magnets coiled with silk-covered wire admit also of better finish, but
for most purposes cotton-covered wire will give satisfaction, especially
if well paraffined. This wire must be wound on the bobbins, from end to
end regularly, with the coils side by side, as a reel of cotton is
wound. This may be done on a lathe, but a little practice will be
necessary before the inexperienced hand can guide the wire in a regular
manner. If, however, the spool of wire have a metal rod passed up its
centre, and this be held in the hand at a distance of a foot or more
from the bobbin on the lathe, the wire will almost guide itself on,
providing the guiding hand be allowed to follow its course. With a
little care, the wire for these little magnets may be wound entirely by
hand. Before commencing to wind on the bobbins, just measure off 8 in.
of the wire (not cutting it off) and coil this length around a pencil,
to form a small coil or helix. The pencil may then be withdrawn from the
helix thus formed, which serves to connect the wire with one of the
points of contact. This free end is to be fastened outside the bobbin by
a nick in the head; or the 1/8 in. length, before being formed into a
helix, may be pushed through a small hole made on the head of the
bobbin, so that 8 in. project _outside_ the bobbin, which projecting
piece may be coiled into a helix as above described. The wire should now
be wound exactly as a reel of cotton is wound, in close coils from end
to end, and then back again, until three layers of wire have been laid
on, so that the coiling finishes at the opposite end to that at which it
began. To prevent this uncoiling, it should be fastened by tying down
tightly with a turn or two of strong silk. The wire should now be cut
from the hank, leaving about 2 in. of free wire projecting at the
finishing end of each bobbin. In cases where many bobbins have to be
wound, either for bells, for relays, or for indicator coils, a device
similar to that illustrated at Fig. 21 A may be employed. This _electric
bobbin winder_ consists in a table which can be stood on a lathe or near
any other driving wheel. Two carriers, C C, somewhat similar to the back
centre and poppet head of a lathe, hollow inside, and furnished with a
spring and sliding piston spindle, stand one at each end of this table.
The sliding spindle of the one carries at its extremity a pulley, A, by
means of which motion can be transmitted from the band of the driving
wheel. The sliding spindles, B B, are fitted with recesses and screws, H
H H H, by means of which the temporary wooden cores, or the permanent
iron cores, of the bobbins can be held while the bobbins are being
wound. The bobbin is placed as shown at D; a flat piece of metal, E,
hinged at G, presses against the bobbin, owing to the spring F. The
centre figure shows details of the carrier, C, in section. At the bottom
is shown the spool of wire on a standard L. The wire passes from this
spot between the two indiarubber rollers, M M, on to the bobbin D.
[Illustration: Fig. 21 A.]
When the bobbins have been wound, they may be slipped over the magnet
cores. They should fit pretty tightly; if they do not, a roll of paper
may be put round the magnet cores, to ensure their not slipping when
the bell is at work. The helix ends of the bobbins should stand
uppermost, as shown at Fig. 22 A. A short length of the lower free ends
of wire (near the base or yoke) should now be bared of their covering,
cleaned with emery paper, twisted together tightly, as shown at Fig. 22
B, soldered together, and any excess of wire cut off with a sharp pair
of pliers. To prevent any chance electrical leakage between this bared
portion of the wire and the iron, it should be carefully coated with a
little melted guttapercha, or Prout's electric glue.
[Illustration: Fig. 22.]
Of course, if the operator has any skill at winding, he may wind both
bobbins with one continuous length of wire, thus avoiding joins, taking
care that the direction of the winding in the finished coils be as shown
at Fig. 22 B; that is to say, that the wire from the _under_ side of one
bobbin, should pass _over_ to the next in the same way as the curls of
the letter [rotated S].
[Illustration: Fig. 23.]
[Illustration: Fig. 24.]
[Illustration: Fig. 25.]
[Illustration: Fig. 26.]
[Illustration: Fig. 27.]
The part that next claims our consideration is the _armature_, with its
fittings. The armature is made out of 5/16 square bar iron, of the best
quality, soft, and well annealed, and filed up smooth and true. The
proportionate length is shown at Figs. 23 and 24; and the size of the
iron for other bells is regulated in the same ratio as that of the
cores. Two methods of making and attaching the springs and hammers are
shown. Fig. 24 shows the section of an armature fitted with back spring
and contact spring in one piece. This is cut out of hard sheet-brass, as
wide as the armature, filed or hammered down to the desired degree of
springiness, then filed up true on the edges. It may be attached to the
iron of the armature, either by soldering, by rivetting, or by means of
two small screws. Rivetting is, perhaps, the best mode, as it is not
liable to shake loose by the vibration of the hammer. The spring at its
shank end may be screwed or rivetted to the bracket. Mr. Edwinson
considers this the better form of contact spring. The other form is made
in two pieces, as shown at Fig. 23, where two strips of hard brass are
cut off, of the width of the armature, and the edges filed. A slot is
then cut in the back end of the armature to receive the two brass
strips, and these are soldered into it. The top strip is then bent back
over the armature to form the contact-spring, the other strip being
soldered or rivetted to a small bracket of angle brass. In either case a
short rod of stout hard brass wire is rivetted or screwed into the free
end of the armature, and to the end of this rod is screwed or soldered
the metal bead, or bob, which forms the hammer or "clapper" of the bell.
The next portion to be made is the contact pillar, or bracket, with its
screw, as shown at Fig. 25. This may either be a short stout pillar of
1/4 in. brass rod, about 1 in. high, tapped on one side to receive the
screw, which should be fitted with a back nut; or it may, as shown in
the figure, be made out of a stout piece of angle brass. The exact size
and length of the screw is immaterial; it must, however, be long enough
to reach (when put in its place behind the contact spring) the spring
itself, and still have a few threads behind the back nut to spare. The
screw should be nicely fitted to the pillar, and the lock nut should
clench it well, as when once the adjustment of the parts is found which
gives good ringing, it is advisable that no motion should take place,
lest the perfection of ringing be interfered with. Some makers use a
"set screw" at the side of the pillar wherewith to hold the contact
screw; others split the pillar and "spring" it against the contact
screw; but, all things considered, the back nut gives the greatest
satisfaction. When the bell is in action, a tiny spark is produced at
every make and break of contact between the contact spring and this
screw. This spark soon corrodes the end of the screw and the back of the
spring if brass alone is used, as this latter rusts under the influence
of the spark. To prevent this, a piece of platinum must be soldered or
rivetted to the spring, at the point where the screw touches, as shown
at Fig. 26, and also at the extremity of the contact screw itself. It is
better to rivet the platinum than to solder it, as the platinum is very
apt to absorb the solder, in which case it rusts quickly, and the
goodness of the contact is soon spoiled, when the bell ceases to ring.
To rivet the platinum piece on to the spring, as shown at Fig. 26, it is
only needful to procure a short length of No. 16 platinum wire, say 1/8
in., then, having drilled a corresponding hole at the desired spot in
the contact spring, put the platinum wire half way through the hole, and
give it one or two sharp blows on an anvil, with a smooth (pened)
hammer.
[Illustration: Fig. 28.]
[Illustration: Fig. 29.]
This will at once rivet it in its place, and spread it sufficiently to
make a good surface for contact. The screw must likewise be tipped with
platinum, by having a small hole bored in the centre of its extremity,
of the same diameter as the platinum wire, which must then be pushed in,
and rivetted by hammering the end, and burring the sides of the screw.
Whichever method be adopted, care must be taken that the platinum tip on
the screw and the speck on the contact spring are adjusted so as to
touch exactly in their centres. It will be hardly worth while for the
amateur to cast or even turn up his own bells (which are generally of
the class known as clock gongs), as these can now be procured so cheaply
already nickelled (see Fig. 28). The bell must be adjusted on its pillar
(see Fig. 29^A), which is itself screwed into a hole in the base-plate,
where it is held by a nut. The adjustment of the bell is effected by
placing it over the shoulder of the pillar, and then clenching it down
by screwing over it one or other of the nuts shown at Fig. 29. The bell
should clear the base, and should be at such a height as to be struck on
its edge by the hammer or clapper attached to the armature, Figs. 23 and
24. We still need, to complete our bell, two binding screws, which may
take either of the forms shown at Fig. 27; and an insulating washer, or
collar, made of ebonite or boxwood, soaked in melted paraffin, to
prevent the contact pillar (Fig. 25) making electrical contact with the
metal base. The best shape to be given to these washers is shown at Fig.
30. They consist in two thin circlets of wood or ebonite, that will just
not meet when dropped, one on the one side, and one on the other of the
hole through which the shank of the contact pillar passes when set up on
the base-plate. If a wooden base be used below the metal base-plate,
then only one washer, or collar, need be used--that is, the one
_above_--since the screw of the pillar will pass into the wood, and this
is not a conductor. If the metal base alone be used, both washers must
be employed, and a small nut (not so large as the washer) used to
tighten up and hold the pillar firm and immovable in its place opposite
the contact spring.
[Illustration: Fig. 30.]
Having now all the parts at hand, we can proceed to fit them together,
which is done as follows:--The bell pillar, with its bell attached, is
fastened by its shank into the hole shown near B, Fig. 17, where it is
screwed up tight by the square nut shown at Fig. 29 _c_. In the same
manner, we must fasten the contact pillar, or bracket, shown at Fig. 24
A. Whichever form be used, we must take great care that it be insulated
from metallic contact with the metal base-plate by washers, as shown at
Fig. 30 (similar washers must be used for the two binding screws if the
_whole_ base-plate be made in metal). This being done, the metal frame,
Fig. 18, is put in position on the wooden base, as shown at Fig. 17, and
screwed down thereto by the screws indicated at _s s s_. The magnet may
then be screwed down to the metal frame as shown. The small bracket of
angle brass marked B, in Figs. 23 and 24, is next screwed into its
place; that is, in such a position that the armature stands squarely
facing the poles of the electro-magnet, but not quite touching them (say
1/16 of an inch for a 2-1/2 in. bell). In setting up this and the
contact pillar, the greatest care must be taken that the platinum tip of
the contact screw, Fig. 25, should touch lightly the centre of the
platinum speck at the back of the spring, Figs. 23 and 24, shown full
size at Fig. 26.
The free ends of the helically coiled electro-magnet wires should now be
inserted into short lengths of small indiarubber tubing (same as used
for feeding bottles), the extremities being drawn through and 1 in. of
the copper wire bared of its covering for the purpose of making good
metallic contact with the connections. One of these ends is to be
soldered, or otherwise metallically connected, to the angle brass
carrying the armature, spring and clapper, the other being similarly
connected with the left-hand binding-screw, shown at Fig. 17. Another
short length of wire (also enclosed in rubber tubing) must be arranged
to connect the contact screw pillar Fig. 17, with the right-hand
binding-screw. When this has been done, we may proceed to test the
working of the bell by connecting up the binding screws with the wires
proceeding from a freshly-charged Leclanch['e] cell. If all have been
properly done, and the connections duly made, the armature should begin
to vibrate at once, causing the "bob," or hammer, to strike the bell
rapidly; that is, provided the platinum tipped screw touches the
platinum speck on the contact spring. Should this not be the case, the
screw must be turned until the platinum tip touches the platinum speck.
The armature will now begin to vibrate. It may be that the clapper runs
too near the bell, so that it gives a harsh, thuddy buzz instead of a
clear, ringing sound; or, possibly, the clapper is "set" too far from
the bell to strike it. In either case a little bending of the brass wire
carrying the clapper (either from or towards the bell, as the case may
dictate) will remedy the defect. It is also possible that the armature
itself may have been set too near, or too far from the electro-magnet.
In the latter case, the clapper will not vibrate strongly enough, in the
former the vibration will be too short, and the clapper may even stick
to the poles of the electros, especially if these have not been
carefully annealed. A little bending of the spring, to or from the
magnets, will remedy these deficiencies, unless the distance be very
much too great, in which case the bending of the spring would take the
platinum tip out of the centre of the platinum speck.
[Illustration: Fig. 31.]
[S] 43. Having thus constructed an efficient electric bell we may proceed
to study its action and notice some of the defects to which it may be
subject. In the first place, if we connect up the bell with the battery
as shown in Fig. 17, viz., the left-hand binding-screw with the wire
proceeding from the carbon of the Leclanch['e], and the right-hand screw
with the wire from the zinc, then, if the platinum tipped screw touches
the platinum speck, at the back of the contact spring, a current of
electricity flows from the left-hand binding-screw all round the coils
of the electro-magnets, passes along the contact spring and platinum
speck, thence to the platinum tipped screw along the short length of
wire to the right-hand binding-screw, whence it returns to the zinc
element of the battery, thus completing the circuit. The current, in
thus passing around the electro-magnet cores, converts them, _pro tem._,
into a powerful magnet (see [S] 13); consequently, the armature, with its
contact spring and hammer, is pulled towards the electro-magnets and at
the same time gives a blow to the bell. Now, if instead of having the
platinum speck attached to a flexible spring, it had been attached
bodily to the rigid iron armature, directly the electro-magnets felt the
influence of the current, the platinum speck would have also been pulled
out of contact with the platinum screw, therefore the electro-magnet
cores would have _immediately_ lost their magnetism (see [S] 13, last five
lines). This would have been disadvantageous, for two reasons: 1st,
because the _stroke_ of the hammer would have been very short, and
consequently the ring of the bell very weak; and, 2nd, because, as even
the softest iron requires some appreciable time for the electric current
to flow round it to magnetise it to its full capacity, it would need a
much greater battery power to produce a given stroke, if the contact
were so very short. The use of an elastic contact spring is, therefore,
just to lengthen the time of contact. But the electro-magnets, even when
the flexible spring is used, do actually pull the platinum speck out of
contact with the platinum screw. When this takes place, the circuit is
broken, and no more current can flow round the electro-magnets, the
spring reasserts its power, and the contact is again made between the
contact screw and contact spring, to be again rapidly broken, each break
and make contact being accompanied by a correspondingly rapid vibration
of the armature, with its attendant clapper, which thus sets up that
characteristic rapid ringing which has earned for these bells the name
of trembling, chattering, or vibrating bells.
[S] 44. From a careful consideration of the last two sections it will be
evident that the possible defects of electric bells may be classed under
four heads: viz., 1st, Bad contacts; 2nd, Bad adjustment of the parts;
3rd, Defective insulation; 4th, Warpage or shrinkage of base. We will
consider these in the above order. Firstly, then, as to bad contacts.
Many operators are content with simply turning the terminal wires round
the base of the binding-screws. Unless the binding-screws are firmly
held down on to the wires by means of a back nut, a great loss is sure
to occur at these points, as the wires may have been put on with sweaty
hands, when a film of oxide soon forms, which greatly lowers the
conductivity of the junction. Again, at the junction points of the wires
with the contact angle brass and contact pillar, some workmen solder the
junctions, using "killed spirits" as a flux. A soldered contact is
certainly the best, electrically speaking, but "killed spirits," or
chloride of zinc, should never be used as a flux in any apparatus or at
any point that cannot be washed in abundance of water, as chloride of
zinc is very _deliquescent_ (runs to water), rottens the wire, and
spoils the insulation of the adjacent parts. If solder be used at any
parts, let _resin_ be used as a flux. Even if any excess of resin remain
on the work, it does no harm and does not destroy the insulation of any
of the other portions. Another point where bad contact may arise is at
the platinum contacts. Platinum is a metal which does not rust easily,
even under the influence of the electric spark given at the point of
contact. Therefore, it is preferred to every other metal (except,
perhaps, iridium) for contact breakers. Platinum is an expensive metal,
the retail price being about 30s. an ounce, and as it is nearly twice as
heavy as lead (Lead 11. Platinum 21.5) very little goes to an ounce. For
cheap bells, therefore, there is a great temptation to use some other
white metal, such as silver, german silver, platinoid, etc.
The tip of the platinum screw may be tested for its being veritably
platinum in the following mode: Touch the tip with the stopper of a
bottle containing aquafortis, so as to leave a tiny drop on the extreme
point of the suspected platinum. If it boils up green, or turns black,
it is _not_ platinum; if it remains unaltered, it may be silver or
platinum. After it has stood on the tip for a minute, draw it along a
piece of white paper, so as to produce a streak of the acid. Expose the
paper for a few minutes to sunlight. If the streak turns violet or pinky
violet, the metal is _silver_; if the paper simply shows a slightly
yellowish streak, the metal is platinum. The tip of the platinum screw
must be carefully dried and cleaned after this trial before being
replaced.
Secondly, as to bad adjustment. It is evident that the magnets and the
armature must stand at a certain distance apart to give the best effects
with a given battery power. The distance varies from 1/24 in. in the
very smallest, to 1/8 in. in large bells. Sometimes (but only in very
badly made instruments) the armature adheres to the poles of the
electro-magnet. This is due to _residual_ _magnetism_ (see [S] 14), and
points to hard or unannealed iron in the cores or armature. As a
make-shift, this defect may be partially remedied by pasting a thin
piece of paper over that surface of the armature which faces the poles
of the electro-magnets. Another bad adjustment is when the platinum
screw does not touch fairly on the centre of the platinum speck, but
touches the spring or the solder. Rust is then sure to form, which
destroys the goodness of the contact. To adjust the contact spring at
the right distance from the platinum screw, hold the hammer against the
bell or gong. The armature should now _just not touch_ the poles of the
electro-magnet. Now screw up the platinum screw until it _clears_ the
contact spring by about the thickness of a sheet of brown paper (say
1/50 of an inch). Let the hammer go, and notice whether the contact
spring makes good contact with the platinum screw. This may be tried by
the Leclanch['e] cell as well, so as to make sure of the character of the
_ringing_. When this has been satisfactorily adjusted the back-nut or
set screw may be tightened, to insure that the vibration of the hammer
shall not alter the adjustment. It sometimes happens that the spring
that bears the armature is itself either too strong (or set back too
far) or too weak. In the former case, the electro-magnet cannot pull the
armature with sufficient force to give a good blow; in the latter, the
spring cannot return the armature, with its attendant contact spring,
back to its place against the platinum screw. To ascertain which of
these two defects obtains, it is only necessary, while the bell is in
action, to press the spring lightly with a bit of wire, first _towards_
and then _away_ from the electro-magnets. If the ringing is improved in
the first case, the spring is too strong; if improvement takes place in
the latter case, the spring is too weak. The third source of inefficient
action, defective insulation, is not likely to occur in a newly-made
bell, except by gross carelessness. Still, it may be well to point out
where electrical leakage is likely to occur, and how its presence may be
ascertained, localized, and remedied. If the wire used to wind the
electro-magnet be old, badly covered, or bared in several places in
winding, it probably will allow the current to "short circuit," instead
of traversing the whole length of the coils. If this be the case, the
magnet will be very weak: the magnet of a 2-1/2-in. bell should be able
to sustain easily a 1 lb. weight attached by a piece of string to a
smooth piece of 1/2-in. square iron placed across its poles, when
energized by a single pint Leclanch['e] cell. If it will not do this, the
insulation may be suspected. If the wire has been wound on the bare
cores (without bobbins), as is sometimes done, bared places in the wire
may be touching the iron. This may be ascertained by connecting one pole
of a bottle bichromate, or other powerful battery, with one of the wires
of the electro-magnet coils, and drawing the other pole of the battery
across the clean iron faces of the electro-magnet poles. If there is any
leakage, sparks will appear on making and breaking contact. Nothing but
unwinding and rewinding with a well covered wire can remedy these
defects. The other points where the insulation may be defective are
between the binding screws and the base, if this be all of metal; or
between the contact spring block and the base, and the contact pillar.
It is also probable (if the connecting wires have not been covered with
indiarubber tubing, as recommended) that leakage may be taking place
between these wires and some portion of the metal work of the base or
frame. This must be carefully examined, and if any point of contact be
observed, a little piece of Prout's elastic glue, previously heated,
must be inserted at the suspected places. With regard to the binding
screws, if they stand on the wooden base, their insulation (unless the
base be very damp indeed) will be sufficiently good; but if the base is
entirely metallic, then ebonite or boxwood washers must be used to
insulate them from contact with the base-plate. With regard to the
contact spring block and the platinum screw pillar, it is _permissible_
that one or the other should not be insulated from the base or frame;
but one or the other _must_ be insulated by means of ebonite or other
insulating washers. Personally, I prefer to insulate both; but in many
really good bells only the platinum screw pillar is thus insulated. Any
such leakage can be immediately detected by holding one pole of a
powerful battery against the suspected binding-screw, or block, or
pillar, and while in this position, drawing the other pole across some
bare iron portion of the frame or metal base. Sparks will appear if
there is any leakage.
The fourth defect--that is, warpage or shrinkage of the base--can only
occur in badly-made bells, in which the entire base is of wood. A
cursory examination will show whether the board is warped or swollen, or
whether it has shrunk. Warping or swelling will throw the electro-magnet
too far from the armature, or "set" the pillar out of place; shrinkage,
on the contrary, will bring the parts too close together and jamb the
magnets, the armature, and the contact pillar into an unworkable
position.
[S] 45. Before quitting the subject of the defects of bells, it may not be
out of place to mention that no bell that is set to do real work should
be fitted up without a cover or case. The dust which is sure to
accumulate, not to speak of damp and fumes, etc., will certainly
militate against good contacts and good action if this important point
be neglected. The cover or case generally takes the form of a shallow
box, as shown at Fig. 32, and may be made from 1/4-in. teak, mahogany,
or walnut, dovetailed together and well polished. It is fastened to the
base in the same manner as the sides of a Dutch clock, by means of
studs, hooks and eyes. At the bottom of the box is cut a slot, of
sufficient width and length to admit the play of the hammer shank.
[Illustration: Fig. 32.]
In the annexed table is given a general idea of the proportion which
should be observed in the construction of bells of different sizes. It
must be noted that if the bells are to be used at long distances from
the battery, rather more of a finer gauge of wire must be employed to
wind the magnets than that herein recommended, unless, indeed, _relays_
be used in conjunction with the bells.
[S] 46.--
TABLE
Showing proportions to be observed in the different parts of electric
bells.
---------+---------+----------+--------+---------+----------
Diameter |Length of|Diameter |Length |Diameter | B. W. G.
of | Magnet |of Magnet | of |of Bobbin| of Wire
Bell. | Cores. | Cores. |Bobbin. | Head. |on Bobbin.
---------+---------+----------+--------+---------+----------
2-1/2" | 2" | 5/16" | 1-3/4" | 3/4" | 24
3 | 2-1/4 | 3/8 | 2 | 7/8 | 24
3-1/2 | 2-1/2 | 7/16 | 2-1/4 | 1 | 22
4 | 2-3/4 | 1/2 | 2-1/2 | 1-1/8 | 22
4-1/2 | 3 | 9/16 | 2-3/4 | 1-1/4 | 20
5 | 3-1/4 | 5/8 | 3 | 1-3/8 | 18
5-1/2 | 3-1/2 | 11/16 | 3-1/4 | 1-1/2 | 16
6 | 3-3/4 | 3/4 | 3-1/2 | 1-5/8 | 16
6-1/2 | 4 | 13/16 | 3-3/4 | 1-3/4 | 16
7 | 4-1/4 | 7/8 | 4 | 1-7/8 | 16
7-1/2 | 4-1/2 | 15/16 | 4-1/4 | 2 | 14
8 | 4-3/4 | 1 | 4-1/2 | 2-1/8 | 14
8-1/2 | 5 | 1-1/16 | 4-3/4 | 2-1/4 | 14
9 | 5-1/4 | 1-1/8 | 5 | 2-3/8 | 14
9-1/2 | 5-1/2 | 1-3/16 | 5-1/4 | 2-1/2 | 14
10 | 5-3/4 | 1-1/4 | 5-1/2 | 2-5/8 | 14
10-1/2 | 6 | 1-5/16 | 5-3/4 | 2-3/4 | 12
11 | 6-1/4 | 1-3/8 | 6 | 2-7/8 | 12
11-1/2 | 6-1/2 | 1-7/16 | 6-1/4 | 3 | 10
12 | 6-3/4 | 1-1/2 | 6-1/2 | 3-1/8 | 10
---------+---------+----------+--------+---------+----------
[Illustration: Fig. 33 A.]
[Illustration: Fig. 33 B.]
[Illustration: Fig. 34.]
[S] 47. We can now glance at several modifications in the shape and mode
of action of electric bells and their congeners. Taking Figs. 33 A and B
as our typical forms of trembling bell, the first notable modification
is one by means of which the bell is made to give a single stroke only,
for each contact with the battery. This form, which is known as the
"single stroke bell," lends itself to those cases in which it may be
required to transmit preconcerted signals; as also where it is desired
to place many bells in one circuit. Fig. 34 illustrates the construction
of the single stroke bell. It differs from the trembling bell in the
mode in which the electro-magnet is connected up to the binding screws.
In the trembling bell, Fig. 33, the circuit is completed through the
platinum screw pillar, to the binding screw marked Z, hence the circuit
is rapidly made and broken as long as by any means contact is made with
the battery, and the binding screws L and Z. But in the single stroke
bell, Fig. 34, the wires from the electro-magnet are connected directly
to the two binding screws L and Z, so that when contact is made with the
battery, the armature is drawn to the poles of the electro-magnet, and
kept there so long as the battery current passes. By this means, only
one stroke or blow is given to the bell for each contact of the battery.
Of course, directly the connection with the battery is broken, the
spring which carries the armature and clapper flies back ready to be
again attracted, should connection again be made with the battery. To
regulate the distance of the armature from the poles of the
electro-magnets, a set screw Q takes the place of the platinum screw in
the ordinary form, while to prevent the hammer remaining in contact with
the bell (which would produce a dull thud and stop the clear ring of the
bell), a stop (_g_) is set near the end of the armature, or two studs
are fixed on the tips of the poles of the electro-magnets. The mode of
adjusting this kind of bell, so as to obtain the best effect, differs a
little from that employed in the case of the trembling bell. The
armature must be pressed towards the poles of the electro-magnets, until
it rests against the stop or studs. A piece of wood or cork may be
placed between the armature and the set screw Q, to retain the armature
in this position, while the rod carrying the hammer or clapper is being
bent (if required) until the hammer just clears the bell. If it touches
the bell, a thud instead of a ring is the result; if it is set off too
far, the ring will be too weak. The armature can now be released, by
removing the wood or cork, and the set screw Q driven forwards or
backwards until the best effect is produced when tested with the
battery. The tension of the armature spring must be carefully looked to
in these single stroke bells. If it is too strong, the blow will be
weak; if too weak, the hammer trembles, so that a clear single stroke is
not obtainable, as the spring _chatters_.
[S] 48. _The continuous ringing bell_ is the modification which next
demands our attention. In this, the ringing action, when once started by
the push,[12] or other contact maker, having been touched, continues
either until the battery is exhausted, or until it is stopped by the
person in charge. The great use of this arrangement is self-evident in
cases of burglar alarms, watchman's alarms, etc., as the continuous
ringing gives notice that the "call" has not received attention. The
continuous ringing bell differs but little from the ordinary trembling
bell. The chief difference lies in the addition of an automatic device
whereby contact is kept up with the battery, even after the "push"
contact has ceased. As it is desirable for the person in charge to be
able to stop the ringing at will, without proceeding to the place where
the "push" stands, so it is not usual to make the continuous ringing
arrangement dependent on the "push," though, of course, this could be
done, by causing it to engage in a catch, which would keep up the
contact, when once made. Continuous ringing bells may be conveniently
divided into two classes; viz., 1st, those in which a device is attached
to the framework of the bell; which device, when once upset by the first
stroke of the bell, places the bell in direct communication with the
battery independent of the "push" or usual contact; and 2ndly, those in
which a separate device is used, for the same purpose. This latter
arrangement admits of the use of an ordinary trembling bell.
[Footnote 12: A "push," of which several forms will hereafter be
described and figured, consists essentially in a spring carrying a stud,
standing directly over, but not touching, another stud, fixed to a base.
The lower stud is connected to one terminal of battery, the spring is
connected to the bell. When the spring is pressed down, the two studs
come into contact, the current flows, and the bell rings.]
[Illustration: Fig. 35.]
[Illustration: Fig. 36.]
Fig. 35 illustrates the action of bells of the first class. In the first
place it will be noticed that there are three binding screws instead of
two, as in the ordinary pattern, one marked C connected as usual with
the carbon element of the battery; another marked L, which connects with
line wire, and a third, Z, connected by means of a branch wire (shunt
wire), proceeding from the zinc of the battery. It will be seen, that
if the battery current is by means of the push caused to flow through
the coils of the electro-magnets, the armature is attracted as usual by
them, and in moving towards them, releases and lets fall the lever
contact, which, resting on the contact screw, completes the circuit
between Z and C, so that the bell is in direct communication with its
battery, independently of the push. Hence the bell continues ringing,
until the lever is replaced. This can be done, either by pulling a
check string (like a bell-pull) attached to an eye in the lever, or by
means of a press-button and counter-spring; as shown in Fig. 36, A and
B.
[Illustration: Fig. 37.]
[Illustration: Fig. 38.]
[Illustration: Fig. 39.]
In continuous ringing bells of the second class, a detent similar to
that shown at Fig. 35 D is used, but this, instead of being actuated by
the electro-magnet belonging to the bell itself, is controlled by a
separate and entirely independent electro-magnet, which, as it may be
wound with many coils of fine wire, and have a specially light spring
for the armature, can be made very sensitive. This second
electro-magnet, which serves only to make contact with a battery, is
known as a _Relay_, and is extensively employed in many cases where it
is desired to put one or more batteries into, or out of circuit, from a
distance. The relay may be looked upon as an automatic hand, which can
be made to repeat at a distant point contacts made or broken by hand at
a nearer one. Fig. 37 shows this arrangement, attached to the same base
board as the bell itself. On contact being made with the push, the
current enters at C, circulates round the cores of the relay, thus
converting it into a magnet. The armature _a_ is thereby pulled to the
magnet, and in so doing releases the detent lever, which falls on the
contact screw, thus at one and the same time breaking the circuit
through the relay, and making the circuit through the bell magnets B B',
back to the battery by Z. A second modification of this mode of causing
an ordinary bell to ring continuously is shown at Fig. 38, the peculiar
form of relay used therewith being illustrated at Fig. 39. Here, the
relay is placed on a separate base board of its own, and could, if
necessary, be thrown out of circuit altogether, by means of a
_switch_,[13] so that the bell can be used as an ordinary bell or
continuous action at will. It will be noticed that the relay has in this
sketch only one core. But the delicacy of the action is not impaired
thereby, as the armature, by means of the steel spring _s_, is made to
form part and parcel of the magnet, so that it becomes magnetised as
well as the core, and is attracted with more force than it would be, if
it were magnetically insulated. The battery current enters by the wires
C and W, passes round the coils of the electro-magnet, and returns by Z.
In so doing it energises the electro-magnet E, which immediately
attracts its armature A. The forward movement of the armature A,
releases the pivoted arm L, to which is attached a platinum-tipped
contact prong P. This, it will be noticed, is in metallic connection
with the pillar P', and with the base, and, therefore, through the wire
W, with the battery. When the arm L falls, the contact prong completes
the circuit to the bell, through the insulated pillar X. The relay is
thus thrown out of the circuit at the same time that the bell is thrown
in. A device similar to those illustrated at Fig. 36 can be employed to
reset the arm L.
[Footnote 13: Described at [S] 61.]
[Illustration: Fig. 40.]
A rather more complicated arrangement for continuous bell ringing is
shown at Fig. 40. It is known as Callow's, and is peculiarly adapted to
ringing several bells from one attachment, etc. Owing to the relay in
this form being wound with two sets of wires, it takes a little more
battery power; but this disadvantage is compensated by its many good
points. The following description, taken from F. C. Allsop's papers in
the _English Mechanic_, will render the working of Callow's attachment
perfectly clear. "When the button of the push P is pressed, the current
in the main circuit flows from the positive pole C of the battery D
through the relay coil _a_, and thence by the wire _d_ and push P, to
the zinc of the battery. This attracts the armature A of the relay R,
closing the local bell circuit, the current flowing from C of the
battery to armature A of the relay R, through contact post _p_, terminal
L of the bell, through bell to terminal Z, and thence by the wire _g_ to
the zinc of the battery. Part of the current also flows along the wire
from the bell terminal L through the relay coil _b_ and switch W, to
terminal Z of the bell, thus keeping the armature of the relay down,
after the main circuit (through the push) has been broken; the bell
continuing to ring until the shunt circuit is broken by moving the arm
of the switch W over to the opposite (or non-contact) side. The bell can
also be stopped by short circuiting the relay, which can be effected by
an ordinary push. It will be seen that more than one bell can be rung
from the same attachment, and the bell can, by moving the arm of the
switch W, be made continuous ringing or not, at will. If the arm of the
switch is moved over to the opposite side to which it is shown in the
figure, the shunt circuit of the bell through the relay is broken, and
the bell will ring only so long as the button of the push is kept in.
This continuous arrangement is very convenient for front doors, etc.,
where trouble is experienced in securing immediate attention to the
summons. Instead of being taken to the switch, as in Fig. 40, the two
wires are taken to a contact piece fixed on the side of the door frame,
and so arranged that when the door is opened, it either short circuits
or breaks the shunt circuit: thus when the push is pressed, the bell
rings until the door is opened, the continual ringing of the bell
insuring prompt attention."
Mr. H. Thorpe, of 59, Theobald's Road, London, has devised a very
ingenious arrangement for the continuous ringing of one or more bells
for a stated period of time. This is shown at Fig. 40 A. It is set in
action by pulling the ring outside the bottom of the core. The bell or
bells then start ringing, as contact is established and kept up. The
novelty lies in the fact that the duration of the contact, and
consequently of the ringing, can be accurately timed from 5 seconds to
30 seconds, by merely inserting a pin at different holes in the rod, as
shown. After the bells have rung the required time the instrument
automatically resets itself.
[Illustration: Fig. 40 A.]
[S] 49. The modifications we are now about to consider, differ from the
ordinary bell, either in the shape or material of the bell itself, the
relative disposition of the parts, or some structural detail; but not
upon the introduction of any new principle. The most striking is
certainly the Jensen bell, which is shown in section at Fig. 41.
[Illustration: Fig. 41.]
According to Mr. Jensen's system of electric bells, the bell may take
any desired form, that of the ordinary church bell being preferred, and
the electro-magnetic apparatus is placed entirely inside the bell
itself. To attain this end the electro-magnetic apparatus must be
compact in form. A single electro-magnet has pole pieces at each end
opposite to which an armature is suspended from a pivot and balanced by
the hammer of the bell. At the back of the armature there may be a make
and break arrangement, whereby a continuous succession of strokes is
effected, or this may be omitted, in which case a single stroke is
given when the contact with the battery is made, or both may be effected
by separate wires, make contact with one wire, and a single stroke is
struck; make it with the other and the current passes through the make
and break and a succession of strokes is heard. When the contact-breaker
is used, it is so arranged that a slight rub is caused at every stroke,
so keeping the contact clean. The flexible break, with the ingenious
wiping contact, is a great improvement over the ordinary screw, which
often becomes disarranged.
The form of the magnet is such that a considerable degree of magnetic
force is caused by a comparatively small battery power. The
electro-magnetic apparatus being within the bell the latter forms a very
effective and handsome shield for the former. Not only can the bell
shield the electro-magnet from wet but the whole of the conducting wires
as well.
The bell may be screwed to a tube through which passes the conducting
wire, which makes contact with an insulated metallic piece in the centre
of the top of the bell. Both the wire and the contact piece are as
completely shielded from the weather as if within the bell itself.
[Illustration: Fig. 42.]
The great point of departure is the discarding of the unsightly magnet
box, and the hemispherical bell (_see_ Fig. 32), and substituting a bell
of the Church type (see Fig. 42), and placing inside it an
electro-magnet specially arranged. The inventors use a single solenoidal
magnet of a peculiar construction, by which the armature is attracted by
both poles simultaneously. By this means less than half the usual
quantity of wire is required, thus reducing the external resistance of
the circuit one half. Moreover the armature, besides being magnetised by
induction, as acted on in the ordinary method of making electric bells,
is by Messrs. Jensen's plan directly polarised by being in actual
magnetic contact by the connection of the gimbal (which is one piece
with the armature) with the core iron of their magnet. It is thus
induced to perform the largest amount of work with the smallest
electro-motive force. Instead of the armature and clapper being in a
straight line attached to a rigid spring, which necessitates a
considerable attractive power to primarily give it momentum, in the
Jensen Bell the armature and hammer are in the form of an inverted [U],
and being perfectly balanced from the point of suspension, the lines of
force from a comparatively small magnetic field suffice to set this
improved form of armature into instant regular vibration. By using a
flexible break and make arrangement instead of the usual armature spring
and set screw (at best of most uncertain action), it is found that a
much better result is attained, and by this device the armature can be
set much nearer the poles of the magnet with sufficient traverse of the
hammer. This is in strict accordance with the law of inverse squares,
which holds that the force exerted between two magnetic poles is
inversely proportionate to the square of the distance between them, or,
in other words, that magnets increase proportionately in their power of
attraction as they decrease in the square of the distance. It will now
be seen why these bells require so little battery power to ring them:
firstly, the armature and hammer are so perfectly balanced as to offer
but little resistance; secondly, the external resistance to the current
is reduced; and thirdly, the best possible use is made of the
electro-magnetic force at disposal.
[S] 50. The next modification which demands attention is the so-called
"Circular bell." This differs from the ordinary form only in having the
action entirely covered by the dome. Except, perhaps, in point of
appearance, this presents no advantages to that. The bells known as
"Mining bells" resemble somewhat in outward appearance the circular
bell; but in these mining bells the action is all enclosed in strong,
square teak cases, to protect the movement, as far as possible, from the
effects of the damp. All the parts are, for the same reason, made very
large and strong; the armature is pivoted instead of being supported on
a spring, the hammer shank being long, and furnished with a heavy bob.
The domes or bells are from 6 inches to 12 inches in diameter, and are
generally fitted with _single stroke_ movement, so as to enable them to
be used for signalling. The hammer shank, with its bob, and the dome,
which stands in the centre of the case, are the only parts left
uncovered, as may be seen on reference to Figs. 43 A and B, where the
exterior and interior of such a bell are shown.
[Illustration: Fig. 43 A.]
[Illustration: Fig. 43 B.]
[Illustration: Fig 44.]
[S] 51. In the "Electric Trumpet," introduced by Messrs. Binswanger, of
the General Electric Company, we have a very novel and effective
arrangement of the parts of an electric bell and telephone together.
This instrument, along with its battery, line and push, is illustrated
at Fig. 44, where A is a hollow brass cylinder, in which is placed an
ordinary electro-magnet similar to Figs. 20 or 20 A. At the front end,
near B, is affixed by its edges a thin disc of sheet iron, precisely as
in the Bell telephone,[14] and over against it, at B, is an insulated
contact screw, as in the ordinary trembling bell. On the disc of sheet
iron, at the spot where the screw touches, is soldered a speck of
platinum. The wires from the electro-magnet are connected, one to the
upper binding screw, the other to the brass case of the instrument
itself, which is in metallic communication with the sheet iron disc.
The return wire from the contact screw is shown attached to the
insulated piece, and is fastened to another binding screw (not visible)
on the base board. When contact is made with the battery, through the
press or push, the magnet becomes energised, and pulls the iron disc or
diaphragm towards it, causing it to buckle inwards. In doing this,
contact is broken with the screw B; consequently the diaphragm again
straightens out, as the magnet no longer pulls it. Again contact is
made; when of course the same round of performances is continuously
repeated. As the plate or diaphragm vibrates many hundreds of times per
second, it sets up a distinctly musical and loud sound wave, not unlike
the note of a cornet-a-piston, or a loud harmonium reed. With a number
of these "trumpets," each diaphragm being duly tuned to its proper
pitch, it would be possible to construct a novel musical instrument,
working solely by electricity. The "pushes" need only take the form of
pianoforte keys to render the instrument within the grasp of any
pianoforte or organ player.
[Footnote 14: See "Electrical Instrument Making for Amateurs." Whittaker
& Co. Second edition.]
[S] 52. Sometimes the gong or "dome" of the ordinary bell is replaced by a
coil spring, as in the American clocks; sometimes quaint forms are given
to the parts covering the "movement," so as to imitate the head of an
owl, etc. But bells with these changes in outward form will not present
any difficulty, either in fixing or in management, to those who have
mastered the structural and working details given in this chapter.