540 
may be dispensed with; thirdly, that the variations in the 
primary, and consequently the strength and period of delivery 
of the secondary currents is perfectly regular ; fourthly, that the 
strength of the currents in the secondary is very great, With a 
26-inch coil by Apps I have obtained a spark about 7 inches in 
length, of the full thickness of an ordinary cedar pencil. But 
for aspark of thickness comparable at least with this, and of 
2 inches in length, an ordinary 4-inch coil is sufficient. 
Owing to the double currents, the appearance of the discharge 
is that of a bright point at each terminal, and a tongue of the 
ellow flame, such as is usually seen with thick sparks from a 
arge coil, issuing from each. This torrent of flame (which, 
owing to the rapidity with which the currents are delivered by 
the machine, is apparently continuous) may be maintained for 
any length of time. The sparks resemble those given by my 
great coil (exhibited in this theatre on Friday, April 13, 1877, 
and described in the Pilosophical Magazine, 1877, vol. iii. 
p. 30) with a large battery-power and with a mercury break ; but 
with that instrument it is doubtful whether such thick sparks 
could be produced at short intervals, or in a rapid shower, as in 
this case. 
In order to contrast the effects of the two methods, I will 
excite the coil, first with a battery, and secondly with the alter- 
nating machine. You will notice that with the battery we can 
obtain either long, bright, and thin sparks, or short and com- 
paratively thick discharges ; but, unless the latter are made very 
short, they occur only at comparatively long and eyen perceptible 
intervals of time. On the other hand, with the alternate ma- 
chine, although the method does not lend itself so readily to the 
production of long and bright sparks, we can produce a perfect 
torrent of discharges more rapid and more voluminous than by 
any other means yet devised. Long bright sparks can, however, 
be obtained by interrupting the flow of the currents from the 
machine, and by allowing only single currents to pass at com- 
paratively long intervals. It may be interesting to know that 
the number of currents given out by the machine, and conse- 
quently the number of discharges issuing from the coil, is no 
less than 35,200, that is, 17,600 in each direction, per minute. 
The number may be determined by the pitch of the note which 
always accompanies the action of an alternate machine. 
A comparison of the two methods may also be made when a 
Leyden jar is used as a secondary condenser. This application 
of the jar is well known as a valuable aid in spectroscopic 
research; and the employment of the alternating machine so 
materially heightens the effects that, judging from some experi- 
ments made in the presence of Mr. Lockyer, and from others of 
a different character in the presence of Prof. Dewar, I am led to 
hope from it a further extension of our knowledge in this direc- 
tion. In order that you may form, at all events, some rough 
idea of the nature of such discharges, I venture, at the risk of 
causing some temporary inconvenience from the noise, to pro- 
ject the spectrum of this spark. 
I will detain you with only one more instance of comparison. 
The ordinary effect of an induction coil in illuminating vacuum 
tubes is well known. The result is usually rather unsteady. 
Several instruments have been devised to obviate this inconve- 
nience, e.g. the rapid breakers described in the Proceedings of 
the Royal Society (vol. xxiii. p. 455, and vol. xxv. p. 547), or 
the break called the ‘‘ Trembleur” of Marcel Deprez (see 
Comptes rendus, 1881, I. Semestre, p. 1283). The use of the 
alternating machine, however, not only gives all the regularity 
in period, and uniformity in current, aimed at in these instru- 
ments, but also at the same time supplies currents of great 
strength. The result is a discharge of great brilliancy and 
steadiness, and it is perhaps not too much to say that the effects 
are comparable to those obtained with Mr. De La Rue’s great 
chloride of silver battery. The configuration of the discharge 
produced in this way can also be controlled by a suitable shunt 
applied to the secondary circuit ; for example, one formed by a 
column of glycerine and water, or the one consisting of a film 
of plumbago spread upon a slab of slate, constructed by my 
assistant, Mr. P. Ward, and here exhibited. 
One test of the strength of current passing through a tube is 
the amount of surface of negative terminal, which it will illu- 
minate with a bright glow. I have here a tube with terminals, 
in the form of rings, each of which would be regarded of ample 
size for currents obtaided in the ordinary way. These are now 
all connected together so as to form one grand negative 
terminal ; and it will be found that with the currents from the 
alternate machine, the whole system is readily illuminated at once, 
NATURE 
[ April 6, 1882 
It should perhaps be here remarked that, while the strength 
of the secondary currents passing through the tube is partly due 
directly to the strength of the primary currents from the machine, 
it is probably also in part due to the rapidity with which the 
secondary currents follow one another. Owing to the latter cir- 
cumstance the column of gas maintains a warmer and more con- 
ductive condition than would prevail if the inteval between the 
discharge was longer ; and in consequence of this a larger por- 
tion of the discharges can make its way through than would 
otherwise be the case. 
Before leaving the instrumental part of my discourse, I desire 
to bring under your notice a modification of the machine which 
we have thus far used for producing, by the intervention of the 
induction coil, currents of high teasion. This consists of a 
machine of the same general construction as the other, but 
having the armatures wound with a much greater number of 
convolutions of much finer wire. The result is a machine giving 
off currents of sufficient tension ‘to effect, by direct action, dis- 
charges through vacuum tubes, and even in air. The currents 
are of course olternate ; but by diminishing the size of one of 
the terminals to a mere point, as well as by other methods 
described elsewhere, it is possible to shut off the currents in one 
direction, leaving only those in the other direction to discharge 
themselves through the tube. I hope on some future occasion 
to give a fuller account of this remarkabfe machine, which has 
only quite recently been completed. 
Returning to the discharge in air, it will be noticed that when 
the terminals are set horizontally, the torrent of thick discharges 
assumes the appearance of a fame, which takes the form of an 
inverted V. This is the result of convection currents due to the 
heat given off by the discharges themselves. The discharges 
are by their nature, as it were, fixed at each end, but within the | 
limits of discharging distance, free to move about and to extend 
themselves in space, especially in their central part. Further, 
it may be observed that the length of the spark which can be 
maintained is greater than that over which it will leap in the 
first instance. The explanation of this is to be sought in the 
fact, that when the sparks follow very rapidly in succession, the 
whole path of each discharge remains so far in a heated state, 
as to assist the passage of the next; and, further, that in the 
middle part of the discharge or apex of the A, where the heat 
is greatest, the heat prevails to such an extent as to render a 
portion of the path highly conductive. This may be illustrated 
by holding a gas jet near the path of the discharge. The flames 
will then leap to the two ends of the jet, which will perform the 
part of a conductor ; and the real length of the discharge will 
be that traversed from terminal to terminal, minus the length of 
the intervening flame. The permanently heated part of the 
flame will act in the same manner in extending the effective 
length of the discharge. 
The discharge which we are now examining is not homo- 
geneous throughout, but consists of more than one layer. The 
flame, which, from the fact of its forming the outer sheath of 
the discharge, is the most prominent feature, consists mainly of 
heated but solid particles emanating from the terminals. That 
this is the case may be inferred in a general way from the colours 
which the flame assumes when different substances are placed 
upon the terminals; for example, lithium or sodium, The 
spectrum of the flame appears to be always continuous. A 
convenient substance to affix to the terminals is boron glass, on 
account of the brilliancy to which it gives rise in the discharge ; 
this will enable us to project the phenomenon. Within this 
sheath of flame, the discharge consists of the ;ink light charac- 
teristic of air, and in the centre of all the true bright spark. 
There is reason to think that, under certain circumstances, there 
are more layers to be seen; but the above division is sufficient 
for our present purpose. In this somewhat complicated struc- 
ture, the pink light corresponds to the arc, and the flame to a 
similar accompaniment which is seen playing about the upper 
carbon in electric lamps when a current of great strength is 
used, 
From this account of the methods here employed I now tura 
to the main question, In the investigation, to which allusion 
was made at the beginning of this lecture, it occurred to us that 
an examination of the effects of a magnetic field on discharges 
of this character through air or other gases at atmospheric pres- 
sure, and a comparison with those obtained at lower pressures, 
might throw some fresh light on the nature of electrical dis- 
charges in general. It is these phenomena to which I now 
propose to ask your attention. 
