May 23, 1913.

THE COLLIERY GUARDIAN.

1065

rocked gradually forward, until they are at the position
where they are performing the work* that is required of
them.

The constant-current motor may be worked in two
ways. It may be given a variable speed, which is
obtained simply by moving the brushes forward and
backward, from the point of normal speed ; or it may be
given a constant speed, the brushes maintaining the motor

at that speed, and dealing with the variation in the load,
by varying their own position. M. Thury has worked
out a very beautiful automatic device for maintaining
a constant-current motor, at constant speed, while
regulating the brushes. It is shown diagrammatically

in fig. 2. The brushes are attached to spindles in the
usual way, mounted on the usual brush-rocker, which
works on ball bearings. It is moved to and fro by a
servo-motor, an apparatus similar to that employed for
the government of water turbines, in which oil under
pressure revolves the spindle in one direction or the
other, the spindle being geared to the brush-rocker.
The pressure of the oil is maintained by a centrifugal
pump, whose speed increases or decreases, with the
speed of the machine, just as a centrifugal governor
does; it acts very much in the same manner as a
centrifugal governor; and any increase of speed of the
motor is immediately met by an increased oil pressure,
delivered to the servo-motor, in the direction tending to
move the brushes, so that a lower pressure is delivered
to the machine. On the other hand, if the motor lowers
its speed, the centrifugal pump follows suit, and a
lowered oil pressure is delivered in the opposite
direction, so that the brushes are moved in the
direction of increased pressure. This is, of course,
the equivalent of the automatic action of the armature
of the ordinary constant pressure motor; but the

Fig. 2.

Diagram of M. Thury's Regulator for a Motor
Running at Constant Speed.

6ATf ANO SHAFT

VALVE

BRUSH ROCKER

GEAR

'OIL

PUMP

ARMATURE COIL	(

IRON

COMMUTATOR SEGMENT' -’ - ' BRUSH
SHORT- CIRCUITING PATH"7

Fig. 3.

Diagram showing the Short-circuiting of a
Coil while under the Brush.

result is to increase or decrease the pressure delivered
to the motor, instead of increasing or decreasing the
current. With increased load, the motor slows slightly;
an increased pressure following, owing to the action of
the oil pump; with a decreased load, the motor slightly
increases its speed, and a lowered pressure is delivered
to it, owing to the action of the centrifugal pump.

The Question of Sparking.

But everyone who has any experience with continuous
current machines will at once raise the question, “ What
about sparking when the brushes are rocked ” ? And
this is one of the most beautiful parts of M. Thury’s
work. There is no sparking at the brushes, no matter
what position they occupy; except, of course, under
circumstances of very great neglect. Machines have
been working in Switzerland and the South of France
for years without any trouble of this kind whatever;
and they have been working at Willesden for some
months, and while an automatic device there is con-
stantly rocking the brushes, there is absolutely no sign
of sparking at all.

Sparking at the brushes of a continuous current
machine is due to the self-induction of the coil that is
passing under the brush, which gives rise to a powerful
current circulating round the coil itself during the period
it is under the brush ; this being followed by a break in
the closed circuit as the leading segment passes from
under the brush. Fig. 3 shows the arrangement
diagrammatically. Again, the coil of a ring-wound

the two segments of the commutator shown are together
under the brush, the coil shown forms a dosed circuit;
and two things happen at the same time. The magnetic
lines of force which the coil has been just previously
creating, return to it, giving rise to pressure within the
coil; and as the electrical resistance of each individual
coil is very small, the current created in it by those
returned lines of force, when the coil is short-circuited

on itself, is very powerful. In addition, there may be ' The « Oldham” Miner’s Electric Safety Lamp. - The
and sometimes is, some action from the magnetic pole (« Oldham ” miner’s electric safety lamp, the general design
which the coil is approaching. In any case, there is a ..of which is shown in the accompanying plate, possesses the
powerful current circulating in the coil during the time 1 following essential parts:_

the two segments to which it is connected are under the

brush; and it is the breaking of this closed circuit which
causes the sparking. Sparking has been reduced in
modern machines by reducing the number of turns of
the coils and by reducing the magnetic lines of force
which each individual coil creates; but with electric
motors running at variable speeds, sparking was only
suppressed when commutating poles were introduced.
Commutating poles, or interpoles as they are often
called, are additional small electro-magnets placed
between the main electro-magnets; and their office is
to neutralise the pressure induced in the coils which are
under the brushes, and to create a small pressure within
the coils in the same direction as will arise in them after
they have passed out from under the brush. The coils
of the commutating poles are always in circuit with the
armature, and therefore any increase in the strength of
the armature current is reflected in the commutator
poles, and the whole thing is self-acting.

With the ordinary constant pressure motor, as
explained above, the variation of the current in the
armature, with the variation of load, rendered the
suppression of sparking very difficult indeed, and j
practically impossible, with variation of speed at the j
same time, until the commutating pole was introduced.1
The current in the armature, as is well known, creates j
a magnetic field of its own, which acts to a certain
extent in opposition to that created by the field magnets,1
the final field being the resultant of the two. Sparkless 1
commutation is obtained to a large extent by making ,
the magnetic field created by the armature, weak com- *
pared with that created by the field magnets ; but with ’
electric motors whose load varies, this again becomes
practically impossible when the speed is varied. Speed
is varied with shunt-wound electric motors, and with .
series-wound motors whose field magnets are shunted,
by weakening the magnetic field created by the field i
magnets, and any increased power that is required is
obtained by increased current in the armature ; the
result being that the magnetic field created by the
armature current comes more and more into competi-1
tion with that created by the field magnets.

In the constant-current motor, one important advan-
tage is obtained from the construction of the machine. ’
The fact that the current in the armature and field
magnets is always the same eliminates the difficulty
mentioned above of providing for the variation in both,1
and considerably lessens the difficulty of neutralising!
sparking. The remainder of the difficulty is overcome
by the construction of the field magnets with reference [
to the armature. M. Thury claims that his field magnets
are so arranged that they act with regard to the coils 1
under the brushes, no matter in what position the
brushes may be, in exactly the same manner as the
commutating poles do—that is to say. no matter where
the brushes may be, nor what the self-induction in the
coils passing under that may be, the magnetic field
produced by the pole pieces to which each coil is subject
is such that the pressure produced by the self-induction ■
of the coil is neutralised, and the coil itself emerges
from under the brush with a slight pressure in the new
direction.	i

The Use of the Constant-current Motor. I

It appears to the writer that the constant-current
motor is particularly adapted for mining work, because
it can be used sparklessly, at both constant speed and at
variable speed; and because it completely does away
with the necessity of starting and regulating resistances,

controllers, &c. All the work that is required in a mine,1
all the variation of speed or variation of load can be
obtained by rocking the brushes. For driving the winding
drum, for instance, the engineman’s lever is merely j
connected to the brush rocker of the motor which is
geared to the winding drum. For driving the fan,1
variation of speed is obtained when required by moving
the brushes forward or backward ; for the motor driving
the coaling screens the same ; the same for pumps, and
so on. For driving coal-cutting machines, the writer
has suggested a special arrangement by which a separate
constant current circuit of only 100 volts pressure would
furnish current for the electric motor, and the motor

'Grimsby Coal Exports.—The export from Grimsby last
week was as follows:—Foreign: To Ahus, 647 tons; Ant-
werp, 558 ; Christiania, 869 ; Dieppe, 907 ; Drammen, 1,672;
Effijerg, 388 ; Gibraltar, 173 ; Gothenburg, 669; Hamburg,
492; Landsorona, 2,190; Malmo, 524; Nakskov, 437;
Narvik, 3,613; Randers, 1,926; Rostcck, 802; Stockholm,
2,275; and Ystad, 3.7’3 ; total, 21,897 tons. Coastal: To
London, 101 tons; and Whitstable, 262; total, 363. Last

itself would be worked by rocking its brushes by means year for the corresponding week the exports were 26,921

APPROVED SAFETY LAMPS.

An Order (No. 296) has been issued by the Home
Secretary, dated March 13, 1913, under section 33 of
the Coal Mines Act, 1911 (1 & 2 Geo. 5, c. 50), approving
the “ Oldham ” miner’s electric safety lamp for use in
mines to which the Act applies. The Schedule, speci-
fying the conditions under which the lamp may be used,
is as follows :—

F I T TED WITH
SINGLE "DOME”
GLASS

(1)	A solid drawn steel case (tinned).

(2)	An electrical accumulator, the terminals of which are

fitted with spiral springs carrying rubbing contacts:
the construction of the accumulator being such as
to prevent escape of the liquid, whatever the
position of the lamp, whilst allowing the escape
of gas generated by chemical action.

(3)	A top screw plug carrying the bulb, protecting glass',

pillars and crown, and forming a fl±me-tight
connection with the case: the protecting glass,
pillars and crown, being of any of the types shown
in the plate.

(4)	A ; bottom screw plug carrying the accumulator and

forming a fl*me-tight connection with the case.

(5)	An efficient locking device of one or other of the

following types, to prevent unauthorised persons
from unscrewing the bottom plug, viz.—

(а)	A brass locking bolt, held in position by a

lead rivet.

(б)	A magnetic lock, so constructed that the

locking bolt can only be withdrawn by
applying the pole of an electro-magnet
against the cover of the lock.

(6)	Provided:

(1)	That the total weight of the lamp is not more

than 5 lb. 4 oz.

(2)	That the strength of material and attachments

throughout the lamp is not less than in the
sample submitted to test on the 7th and
8th January, 1913.

(3)	That the lamp shall be capable of maintaining

a light of not less than 1-candle power all
round in a horizontal plane throughout a
period of not less than 9 hours, and also of
giving a light of not less than 15-candle
power over an arc of 45 degrees in a horizontal
plane.

(4)	That the lamp has been made at the works of

Messrs. Oldham and Son, at Denton, near
Manchester.

(5) That the lamp shall have marked upon it its
name and the name of the maker.

armature is shown, for the sake of simplicity, When of a lever or wheel fixed outside of the enclosing case.

foreign and 2,825 coastal.