FALLOT OXY-ACETYLENE BLAST.--This is similar to the foregoing, utilising oxygen from a cylinder instead of air, and the light is equal to a powerful limelight, and may be considered as an efficient subst.i.tute, though for _long range_ work the shadow before alluded to becomes more noticeable (for optical reasons which need not be here discussed). The Fallot Company also make a special 'Pressure Generator' which can be used instead of a D.A.

Cylinder; but my experience of this so far is that, although perfectly safe, the blast from it is a little unsteady as compared with a cylinder.

LIMES AND ACCESSORIES.--Limes for Optical Lantern work are usually supplied in the form of cylinders, the 'ordinary' size being 7/8 inch in diameter and about 1 inches long, with a hole drilled longitudinally to take the lime pin. Extra large limes up to 2 inches in diameter are supplied for more powerful jets.

So-called 'soft' limes used to be recommended for 'blow-through' jets as giving a better light than 'hard' limes, but the advantage, if any, is very little, and these limes are now very seldom heard of, possibly because 'blow-through' jets themselves are becoming less and less used, and 'soft'

limes will not stand the heat of a mixed or 'Injector' jet for long.

'Hard' limes are turned out of the hardest stone lime, and must be kept in sealed tins until used, as they rapidly disintegrate when exposed to the air. There are one or two quarries known to provide the best lime for lantern purposes, and the various good brands on the market practically have the same origin as regards raw material, though called by different trade names; and the 'Hardazion' (hard as iron) limes, placed on the market some years ago by a well-known wholesale firm, to be countered shortly after by the 'Hardastil' (harder still) brand, are, I take it, legitimate though amusing instances of phonetic advertis.e.m.e.nt.

Even the best of limes is liable to crack under the heat {39} of a powerful jet, and so a pair of lime-tongs should always be provided, and there is nothing better than the simple form shown in Fig. 25, and which is, or should be, sold by all dealers.

[Ill.u.s.tration: FIG. 25.--Lime-tongs.]

SUBSt.i.tUTES FOR LIMES.--A good subst.i.tute for lime, that will give the same light, stand heat equally as well, and _not_ deteriorate if exposed to the atmosphere, has long been sought for, and some of the more recently discovered refractory materials are more or less satisfactory. 'Mabor'

limes, for example, belong to this cla.s.s, and so do some of the 'pastilles,' which before the war came chiefly from France and to a less extent from Germany.

CHAPTER VI

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 a.s.sistance 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) _amperage_ 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 a.n.a.logy of a water pipe, and the _amperage_ 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 acc.u.mulators 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.

Amperes and volts are convertible terms in a sense; that is to say, a current of 10 amperes at 100 volts requires the same horse-power to generate it as one of 5 amperes at 200 volts, or 20 amperes at 50 volts, but they are by no means convertible as regards their _efficient_ use for our purpose. The amperes 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, amperes, and resistance, being connected by the well-known and simple equation C = E / R, C representing the current in amperes, 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 amperes. Whatever method of lighting we use therefore, _enough_ resistance must always be kept in the circuit to ensure {41} that no more current can pa.s.s 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 pa.s.sed 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 amperage 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 spa.r.s.ely 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 amperage 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 amperage 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 acc.u.mulators, 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 acc.u.mulators 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 s.p.a.ce.

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 cla.s.s-room. It was, I believe, made in Germany and was practically un.o.btainable 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 s.p.a.ce 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 pa.s.s, 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 amperes, and for a large hall 10 or 12 amperes.

The simplest form of arc lamp for lantern purposes is the {45} hand-fed type as ill.u.s.trated 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.

[Ill.u.s.tration: 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 ill.u.s.tration 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 a.s.sists the 'cratering' action, while the negative is generally used 'solid,' that is h.o.m.ogeneous 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 ill.u.s.trated in Fig. 26, but may be done by an automatic arrangement, as will be described later.

[Ill.u.s.tration: 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 pa.s.sing, 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 a.s.suming 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 amperes.

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 subst.i.tuting these figures we get--

Current (10 amperes) = 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.