THE ELECTRIC LIGHT
The electric current provides _the_ light for an optical lantern, though it
may take various forms, such as the incandescent glow-lamp in some shape or
other, the comparatively new Ediswan 'Pointolite' lamp, the enclosed arc,
and the open arc. This little book is not a treatise on electricity, but a
few elementary notes may not be out of place, and may be of assistance to
the non-technical lanternist.
The first point then to be considered in adopting the electric light for
the purpose of lantern projection is the character of the supply, and the
information required may be summed up thus: (1) _E.M.F._, _voltage_, or
_tension_ (these three expressions having exactly the same meaning); (2)
_ampèrage_ or amount of current available; (3) whether current is (_a_)
_continuous_, _constant_, or _direct_ (again three words meaning {40} the
same thing), or (_b_) _alternating_. The E.M.F. or tension corresponds to
_pressure_, to use the mechanical analogy of a water pipe, and the
_ampèrage_ to volume, and the voltage of the supply currents in this
country are usually between 100 and 250 volts. Private lighting sets are
frequently as low as 50, and current derived from accumulators may be
anything from a few volts and upwards. _Power_ currents, such as commonly
employed for tramways, &c., are usually about 500 volts, but the use of
these currents for lighting purposes, though practicable, is not to be
advocated.
Ampères and volts are convertible terms in a sense; that is to say, a
current of 10 ampères at 100 volts requires the same horse-power to
generate it as one of 5 ampères at 200 volts, or 20 ampères at 50 volts,
but they are by no means convertible as regards their _efficient_ use for
our purpose. The ampères used multiplied by the number of volts give the
total power consumed in _watts_, and 1000 watts used for one hour represent
1 _unit_ as charged for on our dreaded lighting bills. The current
available from a public supply may be said to be unlimited so far as our
purpose is concerned, and the amount actually used depends only on the
total electrical resistance of our circuit, and this is measured in _ohms_,
the three factors, viz. volts, ampères, and resistance, being connected by
the well-known and simple equation C = E / R, C representing the current in
ampères, E the tension or E.M.F. (electro-motive force) in volts, and R the
resistance in ohms. The total current we _can_ use, however, is limited by
the size of the cable laid on in the building, and this is automatically
safeguarded (or should be) by the _fuses_, which consist, as is generally
known, of thin wires or strips of tin or lead fixed on a fuse board in an
easily accessible place, and which melt directly the current exceeds a safe
amount in ampères. Whatever method of lighting we use therefore, _enough_
resistance must always be kept in the circuit to ensure {41} that no more
current can pass than has been provided for, and in the case of an arc lamp
this usually means a _resistance_ or rheostat being retained in the circuit
in addition to the arc itself, through which the current is passed and
absolutely wasted, though fortunately the waste in money is negligible, and
for reasons to be discussed later such a resistance is necessary with an
optical lantern arc lamp in any case.
In the case of a glow-lamp, the entire resistance is provided by the
filament of the lamp itself, and that is why an ordinary metal or carbon
filament lamp, for say 200 volts, has to be manufactured with an extremely
long and slender, and therefore fragile, filament, while with an ordinary
pocket-torch, which is usually supplied with current from a dry battery of
some 3 or 4 volts only, the filament can be short and thick.
Speaking generally, glow-lamps on a low voltage current can be made more
efficient than on a high one, and are also longer lived for very obvious
reasons; but, on the other hand, the transmission of current over long
distances is cheaper the higher the tension, as for a given number of watts
the ampèrage is less, and therefore smaller cables can be employed. On the
whole, then, currents of 200 to 250 volts have during recent years become
more common than 100, in spite of greater difficulties in making the lamps;
but occasionally one finds a hall where two or more lamps are wired in
_series_, two 100-volt lamps for example being wired together in series on
a 200-volt circuit. If we are using current for our lantern from an
ordinary lamp socket, this is a possibility that must be borne in mind.
The same considerations, viz. the economy of transmitting power at high
tension and of _using_ it at a lower one, have been mainly responsible for
the rapidly increasing number of alternating current circuits now met with,
especially in sparsely populated districts. An alternating current main is
one in which the current reverses its direction, usually in this country
50, but sometimes 60, 80, 90, or even 100 times {42} per second (there
being unfortunately in Great Britain no standard 'Periodicity' or number of
cycles per second), and for technical reasons which need not be entered
upon here, with these alternating currents the tension and ampèrage can be
mutually converted by means of _transformers_, so that current can be
transmitted at so high a tension, for instance, as 10,000 volts, and used
at a voltage of 50 or 100 or whatever is required, the ampèrage available
being increased in inverse ratio as the tension is decreased. The same
ready power of transformation unfortunately does not apply to the
continuous current, or alternating currents would probably never have been
heard of, but as it is they are very common. For glow-lamps it is
immaterial which current is available, but for arc lamps the continuous is
much to be preferred, though both can be utilised.
With these initial remarks, I will now take in order of illumination the
various methods of utilising the electric current for optical lantern work.
THE ELECTRIC GLOW-LAMP.--The ordinary metal filament lamp is not very
suitable for lantern work, the light not being sufficiently concentrated,
but from what has already been said, it will be evident that this method of
lighting is more suitable where currents of low voltage are available.
An extremely good and intense light can be obtained from a comparatively
small battery of accumulators, which can easily be carried in the hand, and
a short and thick metal filament lamp, similar to those supplied with a
powerful electric torch; and this arrangement is actually used to some
extent by travelling lecturers, but the mess and trouble of keeping the
accumulators in order have prevented the method being generally adopted.
When _alternating_ current is available such a lamp will work well with a
transformer to step down the voltage to the required degree, and the
arrangement is simple, cheap, and efficient, and produces a light at least
equal to that from {43} acetylene. In comparatively small halls, where the
current is alternating, this is undoubtedly the best method of working, as
it is simpler than the arc and amply brilliant enough for all practical
purposes.
With the continuous current the problem is not so simple, as transformation
of voltage is not an easy matter, and a glow-lamp on; say, a 200-volt
circuit involves a long and fragile filament, which it is difficult to
arrange in a small space.
Many years ago the Ediswan Company produced a 'Focus' lamp for the purpose,
with the filament arranged in the form of a square grid, and this lamp gave
a light of about 100 candles, and was fairly successful for a small room.
More recently the Osram Company introduced a similar lamp with a metal
filament arranged somewhat in the form of a cone, and this lamp also
sufficed for a small class-room. It was, I believe, made in Germany and was
practically unobtainable during the war. I understand the Osram Company are
at present arranging to manufacture it in this country, but up to the time
of writing it has not made its appearance.
None of these lamps worked direct on a public lighting circuit can be
regarded as really satisfactory, as it has been found impossible so far to
get a _concentrated_ light; the 100-volt lamps have, of course, been
superior to those made for 200 or 250, but they are all for lantern
purposes far behind low voltage lamps, which are really good when a
suitable current can be obtained.
THE POINTOLITE LAMP.--This lamp produced by the Ediswan Company is in
reality a miniature arc with tungsten electrodes in a highly exhausted
vacuum bulb. To attempt a technical description would be beyond the scope
of this book; it will suffice to say that the action depends upon the same
principle as the various wireless vacuum valves or the Coolidge X-ray tube.
This lamp requires a peculiar starting device which is supplied with it,
and gives a good, intense, and concentrated {44} light, not equal to the
ordinary arc or to limelight, but comparing well with any other form of
illuminant. It can only be used with the continuous current.
THE NERNST LAMP.--This lamp at the present moment is practically
non-existent in this country, having been made exclusively in Germany. Also
as recent improvements in metal filament lamps have rendered it almost
obsolete for ordinary lighting purposes, it is, I think, very doubtful
whether it is still manufactured even in that country, and hence I do not
propose to waste space in an extensive description.
It will suffice to say that the lamp consists of one or more straight rods
or filaments of a refractory material, which are semi-conducting to the
electric current when hot, but non-conducting when cold. To commence with
the filament must be heated, and in the lamps as supplied for lantern work
this is usually done by means of a spirit lamp, which can be removed
immediately the current begins to pass, as the filament is thereafter
maintained at a white heat automatically.
A three-filament Nernst lamp gives as much as 1000 candles, but it is
extremely hot, and the light rather diffuse. The filaments are also very
fragile, so on the whole the lamp was never very much in favour; but on the
other hand it consumed very little current, and could be worked from any
ordinary house lighting main, points which led to its adoption in certain
cases.
THE ELECTRIC ARC.--We now come to _the_ light for optical lantern work, the
brightest, the most concentrated, the cheapest, the easiest to work, in
fact, the illuminant which combines all the virtues and but few drawbacks,
but of course requires one indispensable condition, viz. electric current
laid on. This current may be of any voltage from 70-250, or even higher; it
may be continuous or alternating, though the former is to be preferred; and
it requires a cable for _at least_ 5 ampères, and for a large hall 10 or 12
ampères.
The simplest form of arc lamp for lantern purposes is the {45} hand-fed
type as illustrated in Fig. 26. The essential feature is the pair of carbon
rods, the remainder of the apparatus consisting of mechanical adjustments
to 'feed' these as they burn away, and to accurately maintain them in their
proper positions and in the optical centre of the lantern. Just because the
electric arc provides so small and concentrated a light, it is of extreme
importance that the centring should be exact; and hence mechanical
movements are usually provided for this which are unnecessary with other
illuminants.
[Illustration: FIG. 26.--Hand-fed Arc Lamp.]
The whole question of optical adjustments has, however, been left over for
a future chapter, as it more or less applies to whatever illuminant is
used.
The illustration shows a lamp arranged for continuous current, the upper
carbon, which must be connected to the _positive_ wire, being larger than
the lower (the negative), and very slightly behind it. The light from a
continuous current arc lamp comes chiefly from this upper or positive
carbon, {46} which 'craters' as it is used, and this arrangement has the
effect of radiating the light in the direction required (Fig. 27).
The positive carbon is usually of the 'cored' type, that is provided with a
core of softer carbon, as this assists the 'cratering' action, while the
negative is generally used 'solid,' that is homogeneous right through.
The arc has to be 'struck' in the first place by touching the carbons
together for a moment by the mechanical means provided, and then separating
them to the working distance, which is approximately 1/8 inch. They must
then be maintained at that distance by 'feeding' as they slowly burn away,
and this 'feeding' in arc lamps for lantern work is usually done by hand,
as in the lamp illustrated in Fig. 26, but may be done by an automatic
arrangement, as will be described later.
[Illustration: FIG. 27.]
The current is really carried across the arc by _convection_, or in other
words conducted by a bridge of white hot carbon particles, which
continually stream across from the positive carbon to the negative, and
this bridge, while conducting the current, interposes a very considerable
_resistance_ (otherwise it would not of course become hot).
A certain potential or tension is therefore necessary if a given current is
to be maintained, and this potential has to be greater the longer the arc
and also (though not in direct proportion) the smaller the carbons.
When, however, everything is in the best proportion, _i.e._ length of arc,
size of carbons, and current passing, the potential at the arc lamp
terminals required is approximately 45 volts, and this may be taken as a
fixed figure for any current.
The length of arc to give the best results may also be taken {47} as
approximately fixed at 1/8 inch, and the _variable_ factor for different
currents as required is provided by altering the sizes of carbons employed.
The error must not be made, however, of assuming that an E.M.F. of 45 volts
is sufficient to work an arc lamp, as the minimum in practice is at least
65 volts, and 100 or even 200 volts are advantageous.
I have come across more than one private generating installation where the
innocent owner has put in a dynamo for 45 or 50 volts, depending upon some
carelessly written statement that this is sufficient.
_Why_ a higher E.M.F. is required can be simply explained.
Take for instance an average hand-fed arc lamp as used for lantern work and
consuming, say, 10 ampères.
Take also, as a fact, the statement given above that the necessary E.M.F.
at the actual terminals of the arc lamp may be accepted as a constant at 45
volts, and reverting to the equation given on page 40, C = E / R, and
substituting these figures we get--
Current (10 ampères) = E (45 volts) / R (Resistance of Arc).
It is therefore obvious that under these exact conditions the resistance or
back E.M.F. of the arc, as it is termed, must equal 4.5 ohms.
Now suppose the lamp left for a few seconds unattended, while the carbons
are burning away and the arc is lengthening; in a very few moments the
resistance will have increased, owing to the greater distance between the
carbons, and we will suppose it to have become 5 ohms instead of 4.5.
The current passing will now be 45 / 5 = 9 ampères only.
In other words, a very slight lengthening of the arc has reduced the
current, and therefore the light, by 10 per cent.
Not only so, but 45 volts being needed to maintain an arc of {48} normal
length, it is insufficient to maintain a longer one, and in practice the
effect of leaving an arc under these conditions to itself for even a few
seconds is that it _goes out_, to the annoyance of the lecturer and the
confusion of the operator.
It is just _possible_ to work an arc lamp with a total E.M.F. of 45 volts
by giving one's whole attention to it and never taking the hand off the
feeding handle; but in practice no one with any experience would attempt
it. The arc would almost certainly go out several times during the
exhibition.
Now, take an example of a similar arc lamp consuming 10 ampères but worked
from a supply of 200 volts.
Our equation C = E / R must then obviously become
C (10 ampères) = E (200 volts) / Total Resistance (20 ohms).
The resistance of the arc itself being the same as before, viz. 4.5 ohms,
it is obviously necessary to put an _extra_ fixed resistance equal to 15.5
ohms in series with it in order to make up the total of 20 ohms.
_Now_ leave the arc unattended until the resistance of 4.5 ohms has again
become 5 ohms; the only effect is that our current, instead of remaining at
10 ampères, has become 200 / 20.5 or 9.8 nearly, a difference which is
imperceptible.
This is not all, for it is an elementary rule in electrical science that
the total E.M.F. of any circuit distributes itself along that circuit in
proportion to the distribution of resistance.
In other words, our original E.M.F. of 200 volts will so distribute itself
as to reserve, so to speak, an E.M.F. of 45 volts for the arc, while the
resistance of this remains at 4.5 ohms, but directly this resistance
increases, the E.M.F. at the arc lamp terminals automatically rises, and
therefore the actual diminution in current is even less than the figures
above quoted. {49}
Should the arc tend to 'break' or go out, the resistance across it
automatically becomes infinite and the _whole_ 200 volts is at that moment
available to prevent the occurrence.
Under these conditions, therefore, the operator can safely leave the arc
for many minutes at a time. In carrying out experimental work I have often
left the lantern, walked up to the screen, discussed results with a friend,
and walked back, and the arc has shown no signs of misbehaviour whatever.
[Illustration: Fig. 28.--Resistance.]
In practice any current from 100 volts to 250 volts may be considered as
satisfactory for lantern work with a suitable resistance. Less than this
involves feeding the arc rather frequently, and more may give a nasty
shock, should the operator inadvertently touch a live wire, though I have
worked an arc lamp on a current of as much as 500 volts.
The _resistance_ usually consists of a suitable length of wire of high
resistance (Iron, German Silver, or those alloys known as Platinoid,
Eureka, Manganin, Beacon, &c., are most commonly used) wound in spirals on
a frame, and is generally supplied adjustable (Fig. 28), so that more or
less current may be used as desired. These resistances get pretty hot in
use, and care must be taken that they are placed where they cannot scorch
woodwork, &c., and in cases where the lantern is a fixture it is a good
plan to have the resistance bolted up against a wall once and for all. The
resistance may be placed anywhere in the circuit, so long as the current
passes through it, then through the arc lamp (or _vice versâ_), and back to
the other {50} pole of the supply main; it does not matter in the least
whereabouts it comes.
In cases, however, where one pole of the supply main is _earthed_, it is a
good thing to place the resistance in the 'live' side, as this keeps the
arc lamp within 45 volts of earth potential while it is working, to the
comfort of the operator should he touch a terminal or wire, though with an
ordinary lighting main there is no real fear of a dangerous shock in any
case.
The _amount of current required_ depends of course on the size of the
sheet, length of the hall, and density or otherwise of the slides; but it
is usually accepted in practice that the efficient light from a continuous
current arc lamp equals 100 candles per ampère, and therefore a 10-ampère
arc will give 1000 candles. This is sufficient for all ordinary halls and
slides, but where these latter are very dense, as for example with the
Lumière three-colour process, as much as 20 or 25 ampères may be required.
In these cases some special precautions must be taken for keeping the
slides cool, or the result may be disastrous, but this is a question that
will be referred to in a later chapter. A current of 10 ampères is pretty
safe for all ordinary slides, and may be taken as the normal current used
in large halls, though in arranging for the wiring it is as well to
stipulate for at least 12 or even 15 ampères, especially as there must
necessarily be a momentary increase of current at the instant the arc is
'struck.'
VARIETIES OF HAND-FED ARC LAMPS.--The pattern of hand-fed arc lamp
illustrated in Fig. 26 is only typical of many of the same general design,
and there are others in which the design itself is fundamentally different.
Of these the 'Scissors' arc lamp made by several firms deserves mention on
account of its simplicity and cheapness. As its name implies, the mechanism
resembles a pair of scissors, the carbons being attached to the ends of a
pair of levers hinged together {51} (Fig. 29). In this lamp centring
movements are usually dispensed with, the arc being clamped on to a tray
pin as in the case of a limelight jet. This is not, of course, so
convenient, and a further disadvantage of this pattern arc lamp is that the
feeding process gradually alters the position and angle of the carbons. In
fact, the one great merit of the lamp is cheapness, and where expense is an
object, it should certainly be considered.
[Illustration: FIG. 29.--'Scissors' Arc Lamp.]
Yet another arc lamp deserving of mention is the 'Parallel,' a name again
very aptly chosen, as the two carbons are either exactly parallel to each
other or very slightly inclined. In the former case the arc has to be
'struck' by touching the ends of the carbon rods with a piece of metal or
carbon. Of the actual manipulation of this lamp I have had very little
practical experience, but I have heard it well spoken of, though I believe
it has so far only been made for currents of 5 ampères or so.
Yet another type which must not be ignored is the 'Right-angled' pattern
(Fig. 30), a name again self-descriptive. The horizontal carbon is the
positive, and the vertical the {52} negative, and this lamp again is made
by several manufacturers in slightly different forms.
This pattern lamp is in my experience the best of all for _small_ currents,
say, of 5 ampères or so, but inferior to Fig. 26 for currents of 10 ampères
or more. This last remark perhaps hardly applies to _alternating_ currents,
which, however, I have not yet discussed. I cannot conclude this brief
category of arc lamps without referring to the _enclosed_ pattern, of which
the 'Westminster' is perhaps the best-known and most popular (Fig. 31).
[Illustration: FIG. 30.--'Right-angled' Arc Lamp.]
This is a lamp of the right-angled type, but the arc burns in a cylindrical
glass chamber, not air-tight, but partially so. After burning a few minutes
the oxygen in this chamber becomes used up and its place is taken by
carbonic-acid gas and other products of combustion, after which the carbons
burn away very much more slowly, and therefore require feeding at much
greater intervals.
This lamp again is chiefly made for small currents not exceeding 5 ampères
(and can therefore be used from any ordinary lamp socket), and for a
moderate-sized hall is on the {53} whole as cheap, efficient and simple a
lamp as any I am acquainted with. It can be supplied with or without
mechanical centring movements as required, and is usually sent out with its
own resistance for the particular current on which it is to be used, so
that it only requires connecting up to the nearest lamp socket, and is
ready for use.
[Illustration: FIG. 31.--'Westminster' Arc Lamp.]
It is _not_ sufficient for anything larger than a 12-foot sheet or for
working at a greater distance than, say, 40 feet, but within these limits
the lamp, and in fact _any_ good 5-ampère arc lamp, will be found quite
satisfactory and saves the expense of putting in a special cable.
AUTOMATIC ARC LAMPS.--Arc lamps for lantern work in which the feeding is
done automatically are also made. Like hand-fed lamps, they vary in exact
design, but all, or practically all, are so designed that the carbons are
brought together by means of springs or weights, and some form of 'brake'
controlled by a system of electro-magnets checks the {54} movement. As the
carbons burn away the arc lengthens, the current weakens, the
electro-magnets lose their grip, and the carbons move together until the
increasing current puts on the brake again. Some of these lamps are
'semi-automatic' only, that is to say, the arc has to be struck by hand,
while others perform this operation automatically as well, usually by an
additional magnet which draws back the carbons by the correct amount after
the arc is struck.
My frank advice to intending lanternists is to leave these lamps alone.
Some of them are satisfactory up to a point, but they are all apt to be
'jumpy,' and on the whole the hand-fed type is in my opinion to be
preferred.
ARC LAMPS ON ALTERNATING CURRENTS.--The alternating current is not so good
as the continuous for lantern work with arc lamps: the light per ampère is
not so great, the light has an irritating habit of travelling round the
carbons and there is always a slight 'hum.'
The sum total of these drawbacks is nothing very serious, provided that
proper arrangements are adopted, and I have frequently manipulated arc
lamps on alternating circuits with such good results that professional
lecturers have at first refused to believe that the circuit really _was_
alternating.
As it is frequently stated that to obtain a steady light with an
alternating current is impossible, I can understand their surprise, and I
can also understand the statement in question, as the problem is usually
tackled on entirely wrong lines.
It is almost always stated that arc lamps for alternating currents should
be arranged with the carbons _vertical_, and many makers actually so
construct their lamps as to allow of this.
To obtain a steady light under these conditions _is_ impossible and I pity
anyone who attempts it; but the statement that this is the best method of
working has been repeated so often that it seems to have been taken for
granted.
The best arrangement (in my hands at any rate) is to {55} slant the carbons
as for the continuous current, and also to have the upper carbon cored and
the lower one solid, but to use a rather larger lower carbon than would be
correct if the main were continuous.
Also the upper carbon should not be _quite_ so far back as with D.C.; to
have the front edges of the two carbons practically in line is about
correct, but the _exact_ position should be carefully adjusted to obtain
the steadiest light, and it will be found that a slight alteration makes a
considerable difference.
It is also a great help to have a weak electro-magnet, or its equivalent,
so arranged that it tends by its influence to keep the arc to the front. On
some lamps this is provided for, as even with a continuous current it is
quite harmless and, if anything, beneficial; but, if not, any competent
mechanic can easily fit an 'Induction Ring,' consisting of a single turn of
stout copper wire, which has sufficient magnetic influence to do all that
is required (Fig. 32).
This ring must be wired in series with the arc itself, and as the current
passing in it automatically reverses in synchronism with the arc, its
effect is _always_ to deflect the arc in the same direction, and care must
of course be taken that it is so wired that the deflection is forward and
not backward. This is the exact arrangement I have myself adopted, and I
never experience any difficulty on the score of the arc wandering.
Right-angled arc lamps, as described on pages 52 and 53, are also very
efficient on A.C. mains, and frequently these lamps are already equipped
with electro-magnets for the purpose required. The 'hum' of an alternating
current cannot be altogether eliminated, but can be reduced to a minimum
_by reducing the voltage as far as possible_.
As has been already said, the A.C. lends itself readily to transformation
of voltage, and I find in practice 90-100 to {56} be ideal. More than this
is inclined to be noisy, and less is apt to result in an unsteady arc.
The arrangement, therefore, which I recommend from long experience is to
employ a transformer to reduce the E.M.F. to 100 volts or thereabouts, and
then work with a resistance in the usual way (if the original current is
100 volts, of course _no_ transformer is required) with a properly
constructed arc lamp fitted with an induction ring or electro-magnet. No
difficulty should then be experienced in obtaining a good, steady, and
fairly quiet light.
[Illustration: FIG. 32.--Arc Lamp with Induction Ring.]
Any little 'hum' remaining can be silenced to a very considerable extent by
placing the entire lantern on a thick block of saddlers' felt, but in
practice I have never found this necessary with ordinary currents, though a
few abnormal circuits where the 'periodicity' is very high are noisier than
others.
[Illustration: FIG. 33.--THE OPTICAL SYSTEM OF A LANTERN.]
{57}
The following table gives the sizes and particulars of carbons for various
currents that I have found best in actual practice:
CONTINUOUS CURRENT
Ampères. + Carbon _Cored_. - Carbon _Solid_.
7-10 12 mm. 7 mm.
10-15 13 ,, 8 ,,
15-20 16 ,, 10 ,,
ALTERNATING CURRENT
Ampères. Upper Carbon _Cored_. Lower Carbon _Solid_.
7-10 12 mm. 10 mm.
10-15 13 ,, 11 ,,
15-20 16 ,, 13 ,,
* * * * *