Oct. 6, 1881] 



NATURE 



547 



shortening the wire, although the explanation of the phenr menon 

 is not quite so simple. If the same process be carried further, 

 the platinum will become white-hot, and if it be carried still 

 further, the platinum will be fused. The Swan, the Maxim, the 

 Lane-Fox, and the Edison lamps, in which the light is due to 

 the incandescence of a fine thread of carbon, are beautiful 

 instances of the application of this principle. 



The platinum, which does not allow the electricity to pass 

 along it with the same facility as the copper, is said to ofifer 

 " greater resistance " than the copper of the same thickness to 

 the passage of the current ; and if we were to measure by a 

 suitable instrumeijt the quantity of electricity which pas-ed 

 through the circuit when the platinum was interposed, and were 

 to compare it with that which passed without the platinum, we 

 should find that the quantity was diminished by the interposition 

 of the platinum. The energy which, as electricity, disappears in 

 its passage throngh the platinum is, however, not really lost, but 

 reappears in the form of heat. 



Instead, however, of interposing in the circuit a length of 

 resisting metal, we may break the circuit altogether, or (to 

 express the same thing in different words) we may interpose an 

 interval of air. In such a case the electricity will no longer flow 

 freely as it does through copper, or even push its way as it does 

 through platinum, but itwilltraver.e the interval only in a diruptive 

 manner in the form of a flash or spark ; and it is to be noted 

 that the interval over which the passage can he made to pass, 

 or length of spark, does not depend, at least in a direct manner, 

 on the quantity of electricity employed or " strength of 

 current," but rather upon the quality of it. This quality is 

 called "tension," and it is measured by the strength of current 

 which it can maintain, 01 cause to flow, through a given resist- 

 ance. The force called into play in the process is called 

 "electro-motive force." Without attempting to go fully into 

 the subject, we may illustrate the relation of quantity or strength 

 of current to teu'^ion or electro motive force in general terms by 

 reference to ihe instrumental means requisite for their ] roduc- 

 tion. Thus it is usually stated that in a battery the quantity 

 depends upon the size of the plates employed, and the tension 

 upon the number of cells ; and similarly, that in a magneto- or 

 a dynamo-machine, the quantity depends mainly on the thickness 

 of the wire used in its construction, and the tension upon the 

 number of convolutions or length of the wire in the culls for 

 a given speed of working, or for a given number of convolution.s, 

 upon the speed at which the m.ichine is driven. 



In further explanation of this, however, it should be pointed 

 out that the current generated has, independently of the external 

 circuit, to pass through the cells of the battery, or through the 

 wires of the machine, both of which offer resistance. When a 

 strong current is required, this resistance may 1 e diminished by 

 increasing the size of the plates in the case of the battery, or by 

 increasing the diameter of the wires in that of the machine. In 

 the lattei case it must be borne in mind this increase in diameter 

 usually involves a diminution of length on account of the neces- 

 sary limitations in the dimensions of the machine, and conse- 

 quently also of electro-motive fcrce. This must be compensated 

 either by increasing the speed of the machine or by augmenting 

 the strength of the field magnets. 



With the Holtz machine the matter is a little different. The 

 quantity of electricity produced depends on the amount of 

 surface of the revolving plates passing in front of the collectors 

 in a given time, and consequently for a given machine upon the 

 speed at which it is driven. Thus there is nothing either in the 

 construction of the machine nor in its internal working which 

 can alter anything except the quantity of electricity producfd, 

 and we must therefore look to the circumstances and mode of 

 discharge for a determination of the tension of the electricity 

 evolved. 



The induction-coil is an instrument for producing from 

 currents of large quantity and low tension others of high 

 tension, but of small quaniity. It consi-ts mainly of tuo 

 parts, viz. a primary coil of thick wire and few convolution^, 

 through which intermittent cuiTcnts ai-e sent from a liatteiy 

 or machine ; and a sec ndary coil outside, but not connected 

 with the former, of fine wire and many convolutions, through 

 which by a kind of sympathetic or "inductive" action 

 temporary cuiTents are set up every time a cmxent begins or 

 ceases in the primary. The tension of the induced currents 

 depends fundamentally upon the length of wire or number of 

 the convolutions in the secondary coil. There are several other 

 parts of the instrument which are important for its working. 



which, however, it is not necessary for oiu: present purpose to 

 particularise. 



From this digi-ession we may now return to our main subject ; 

 •and taking it up again at the point where we left it, viz. the 

 heating of resisting metals, we may vaiy the experiment by 

 taking a piece of iron whe, and bringing to bear upon it some 

 of the induced high tension currents from the induction-coil. It 

 will now be found that if the sparks follow one another with 

 sufficient rapidity, the w ire will not have time to cool during the 

 interval between two successive sparks, and that it will burn 

 like a match or other combustible substance. 



If, however, we use, instead of iron, some metal very difficult 

 of fusion, or " ref ractoi-y, " as it is called, such as iridium, the 

 consumption of material will be extremely .small; and in the 

 incandescent terminals v\e shall have a source of light of con- 

 siderable power. And further, if the terminals be inclosed in 

 an envelope impervious to air, and either well exhausted or 

 partially filled with suitable gas other than oxygen, nitrogen for 

 example ; then the loss by oxidation will be reduced to an in- 

 significant amount. On this principle Mr. Gordon has con- 

 structed a lamp, which consequently has, at all events, the 

 scientific interest of occupying a position intermediate between 

 the incandescent and the arc lamps. 



La.-tly, if we accumulate a large quantity of electricity in a 

 Leyden jar, and discharge it all at once through a thin wire or 

 film of badly conducting met.il, we shall cause the metal (in this 

 case a strip of gold leaf) to be not only fused but to be shattered 

 or deflagi-ated, in the manner which you will immediately see. 

 The image of the gold leaf is now thrown on the screen, the jar 

 is char!i;ed by currents from the induction coil, and is discharged 

 througli the metal. The gold leaf is now shattered by the 

 passage of a high tension charge, the quantity of which is greater 

 than it can carry ; and in the image of its remains we may trace 

 indications of the forces which have been at work in the process 

 of destruction. Observe, in particular, how the particles have 

 been thrown laterally outwards, as if by an explosion from 

 inside the gold leaf. In the alternations of range of the laterally 

 scattered particles Mr. De la Rue traces an analogy to the 

 phenomena of striation described below. And if these alterna- 

 tions are not due to diversities in the conducting power of the 

 wire at various points, but to resistances set up periodically by 

 the discharge itself in its passage, the two phenomena must 

 certainly have something in common. 



I do not, how ever, propose to pursue these forms into greater 

 detail, because the subject to which I wish more particularly to 

 draw your .atteniion, as the most fruitful both in results actually 

 obtained and in promi-e for the fuiure, is the passage of the 

 discharge through air and other gases. And I h.ive adduced 

 these experiments wiih metallic -substances in order to show that 

 the discharge through them is capable of various modifications, 

 analogous to those which we shall presently see in gaseous 

 media. 



Turning then our attention to gases, it w ill be convenient, for 

 instrumental and other reasons, to invert the order of experi- 

 ments, so as to begin with the form of discharge which corre- 

 sponds to the deflagration experiment, and to proceed thence to 

 less violent forms. 



We will now make use of the Holtz machine. If, while the 

 instrument is in action, we separate the terminals to any mode- 

 rate distance, the discharge will take the form of a bright spark 

 extending usually in an irregular line from one terminal to the 

 other, if, instead of di-charging the machine or coil in this 

 manner, we charge a Leyden jar, and then discharge it ; or if, 

 what is sul stantially the same thing, we insert a Leyden jar in 

 Ihe circuit, allowing it to become charged and to discharge itself, 

 then the discharge is of a character similar to that above de- 

 scribed, except that it is shorter in span, and at the same time 

 more brilliant in illumination. This is due to the greater quan- 

 tity of electricity discharged at once. It is moreover to be 

 observed that, however great the quantity of electricity passirg 

 in this manner, the di-charge appears to be absolutely instanta- 

 neous. It is moreover a curious circumstance, attested by many 

 experiments, that the form of discharge in which a Leyden jar 

 is used appears to be incompetent of itself to communicate heat 

 to even inflammable bodies. Thus, such a discharge will pierce 

 a card without leaving any signs of charring behind ; and it w ill 

 disperse a heap of gunpowder, through which it passes, like a 

 heap of sand, without exploding it. It may be added that gun- 

 cotton itself, even in a state favourable to explosion, when ex- 

 posed to a discharge of this kind, is not only not ignited, but 



