[Footnote: Paper read at the British Association, Southampton. Revised
by the Author.--_Nature_.]
By W.H. PREECE.
Electric lamps on the arc principle are almost as numerous as the trees
in the forest, and it is somewhat fresh to come upon something that is
novel. In these lamps the carbons are consumed as the current flows, and
it is the variation in their consumption which occasions the flickering
and irregularity of the light that is so irritating to the eyes. Special
mechanical contrivances or regulators have to be used to compensate for
this destruction of the carbons, as in the Siemens and Brush type, or
else refractory materials have to be combined with the carbons, as in
the Jablochkoff candle and in the lamp Soleil. The steadiness of the
light depends upon the regularity with which the carbons are moved
toward each other as they are consumed, so as to maintain the electric
resistance between them a constant quantity. Each lamp must have a
certain elasticity of regulation of its own, to prevent irregularities
from the variable material of carbon used, and from variations in the
current itself and in the machinery.
In all electric lamps, except the Brockie, the regulator is in the lamp
itself. In the Brockie system the regulation is automatic, and is made
at certain rapid intervals by the motor engine. This causes a periodic
blinking that is detrimental to this lamp for internal illumination.
[Illustration: FIG. 1. FIG. 2.]
M. Abdank, the inventor of the system which I have the pleasure of
bringing before the Section, separates his regulator from his lamp.
The regulator may be fixed anywhere, within easy inspection and
manipulation, and away from any disturbing influence in the lamp. The
lamp can be fixed in any inaccessible place.
_The Lamp_ (Figs. 1, 2, and 3.)--The bottom or negative carbon is fixed,
but the top or positive carbon is movable, in a vertical line. It is
screwed at the point, C, to a brass rod, T (Fig. 2), which moves freely
inside the tubular iron core of an electromagnet, K. This rod is
clutched and lifted by the soft iron armature, A B, when a current
passes through the coil, M M. The mass of the iron in the armature is
distributed so that the greater portion is at one end, B, much nearer
the pole than the other end. Hence this portion is attracted first, the
armature assumes an inclined position, maintained by a brass button, t,
which prevents any adhesion between the armature and the core of the
electromagnet. The electric connection between the carbon and the coil
of the electromagnet is maintained by the flexible wire, S.
[Illustration: FIG. 3.]
The electromagnet, A (Fig. 1), is fixed to a long and heavy rack, C,
which falls by its own weight and by the weight of the electromagnet and
the carbon fixed to it. The length of the rack is equal to the length of
the two carbons. The fall of the rack is controlled by a friction break,
B (Fig. 3), which acts upon the last of a train of three wheels put
in motion by the above weight. The break, B, is fixed at one end of
a lever, B A, the other end carrying a soft iron armature, F,
easily adjusted by three screws. This armature is attracted by the
electromagnet, E E (whose resistance is 1,200 ohms), whenever a current
circulates through it. The length of the play is regulated by the screw,
V. The spring, L, applies tension to the break.
_The Regulator_.--This consists of a balance and a cut-off.
_The Balance_ (Figs. 4 and 5) is made with two solenoids. S and S',
whose relative resistances is adjustable. S conveys the main current,
and is wound with thick wire having practically no resistance, and S'
is traversed by a shunt current, and is wound with fine wire having a
resistance of 600 ohms. In the axes of these two coils a small and light
iron tube (2 mm. diameter and 60 mm. length) freely moves in a vertical
line between two guides. When magnetized it has one pole in the middle
and the other at each end. The upward motion is controlled by the
spring, N T. The spring rests upon the screw, H, with which it makes
contact by platinum electrodes. This contact is broken whenever the
little iron rod strikes the spring, N T.
The positive lead from the dynamo is attached to the terminal, B, then
passes through the coil, S, to the terminal, B', whence it proceeds to
the lamp. The negative lead is attached to terminal, A, passing directly
to the other terminal, A', and thence to the lamp.
[Illustration: FIG. 4]
The shunt which passes through the fine coil, S', commences at the
point, P. The other end is fixed to the screw, H, whence it has two
paths, the one offering no resistance through the spring, T N, to the
upper negative terminal, A'; the other through the terminal, J, to the
electromagnet of the break, M, and thence to the negative terminal of
the lamp, L'.
[Illustration: FIG. 5.]
_The Cut-off_.--The last part of the apparatus (Fig. 4) to be described
is the cut-off, which is used when there are several lamps in series. It
is brought into play by the switch, C D, which can be placed at E or D.
When it is at E, the negative terminal, A, is in communication with
the positive terminal, B, through the resistance, R, which equals the
resistance of the lamp, which is, therefore, out of circuit. When it is
at D the cut-off acts automatically to do the same thing when required.
This is done by a solenoid, V, which has two coils, the one of thick
wire offering no resistance, and the other of 2,000 ohms resistance. The
fine wire connects the terminals, A' and B. The solenoid has a movable
soft iron core suspended by the spring, U. It has a cross-piece of iron
which can dip into two mercury cups, G and K, when the core is sucked
into the solenoid. When this is the case, which happens when any
accident occurs to the lamp, the terminal, A, is placed in connection
with the terminal, B, through the thick wire of V and the resistance, R,
in the same way as it was done by the switch, C D.
_Electrical Arrangement_.--The mode in which several lamps are connected
up in series is shown by Fig. 6. M is the dynamo machine. The + lead is
connected to B1 of the balance it then passes to the lamp, L, returning
to the balance, and then proceeds to each other lamp, returning finally
to the negative pole of the machine. When the current enters the balance
it passes through the coil, S, magnetizing the iron core and drawing
it downward (Fig. 4). It then passes to the lamp, L L', through the
carbons, then returns to the balance, and proceeds back to the negative
terminal of the machine. A small portion of the current is shunted off
at the point, P, passing through the coil, S', through the contact
spring, T N, to the terminal, A', and drawing the iron core in
opposition to S. The carbons are in contact, but in passing through
the lamp the current magnetizes the electromagnet, M (Fig. 2), which
attracts the armature, A B, that bites and lifts up the rod, T, with the
upper carbon, a definite and fixed distance that is easily regulated
by the screws, Y Y. The arc then is formed, and will continue to burn
steadily as long as the current remains constant. But the moment the
current falls, due to the increased resistance of the arc, a greater
proportion passes through the shunt, S' (Fig. 4), increasing its
magnetic moment on the iron core, while that of S is diminishing. The
result is that a moment arrives when equilibrium is destroyed, the iron
rod strikes smartly and sharply upon the spring, N T. Contact between T
and H is broken, and the current passes through the electromagnet of the
break in the lamp. The break is released for an instant, the carbons
approach each other. But the same rupture of contact introduces in the
shunt a new resistance of considerable magnitude (viz., 1,200 ohms),
that of the electromagnets of the break. Then the strength of the shunt
current diminishes considerably, and the solenoid, S, recovers briskly
its drawing power upon the rod, and contact is restored. The carbons
approach during these periods only about 0.01 to 0.02 millimeter.
If this is not sufficient to restore equilibrium it is repeated
continually, until equilibrium is obtained. The result is that the
carbon is continually falling by a motion invisible to the eye, but
sufficient to provide for the consumption of the carbons.
[Illustration: FIG. 6]
The contact between N T and H is never completely broken, the sparks are
very feeble, and the contacts do not oxidize. The resistances inserted
are so considerable that heating cannot occur, while the portion of the
current abstracted for the control is so small that it may be neglected.
The balance acts precisely like the key of a Morse machine, and the
break precisely like the sounder-receiver so well known in telegraphy.
It emits the same kind of sounds, and acts automatically like a skilled
and faithful telegraphist.
This regulation, by very small and short successive steps, offers
several advantages: (1) it is imperceptible to the eye; (2) it does not
affect the main current; (3) any sudden instantaneous variation of the
main current does not allow a too near approach of the carbon points.
Let, now, an accident occur; for instance, a carbon is broken. At once
the automatic cut-off acts, the current passes through the resistance,
R, instead of passing through the lamp. The current through the fine
coil is suddenly increased, the rod is drawn in, contact is made at G
and K, and the current is sent through the coil, R. As soon as contact
is again made by the carbons, the current in the coil, S, is increased,
that of the thick wire in V diminished, and the antagonistic spring,
U, breaks the contact at G and K. The rupture of the light is almost
invisible, because the relighting is so brisk and sharp.
I have seen this lamp in action, and its constant steadiness leaves
nothing to be desired.
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