April 6, 1882] 
NATURE 
541 
When the discharge, originally in the form of a vertical 
Spindle, is submitted to the action of a magnet whose poles are 
horizontal, it spreads out into two nearly semicircular disks, one 
due to the discharges in one direction, and the other to those in 
the opposite direction, As the magnetism is strengthened, the 
flame retreats towards the edge of the disks, and ultimately dis- 
appears. 
but with a still stronger magnetic field, it is traversed at intervals 
by bright semicircular sparks at various distances from the centre. 
In eyery case, bright sparks pass directly between the terminals 
at the opening of each separate discharge. 
SS 
The disk then consists mainly of the pink discharge ; | 
In order further to disentangle the parts of this phenomenon, 
recourse was had in the original experiments to a revolving 
mirror. The light in the disks is insufficient to allow of a 
projection of the effects, but the accompanying diagrams re- 
| present the appearances seen in the mirror. Fig. 1 shows the 
arrangement of the terminals and the magnetic poles; Fig. 2 
the appearance of the discharges in a plane at right angles to 
that of Fig. 1; Fig. 3 the appearance of three successive dis- 
charges (in the same direction) with a weak magnetic field and 
a slowly revolving mirror; Fig. 4 the same, with a slightly more 
| rapid rate of revolution; Fig. 5 a single discharge, with a 
sae Se . 
N i) 
N 
—~ 221 
Fic. 1. 
stronger; field and greater speed of mirror ; Fig. 6 .a single dis- 
charge in a strong field, with a still greater speed of mirror. It 
should be mentioned, that in all these figures the images to the 
left areto be regarded as anterior to those on the right, and 
that they represent various phases of the left-hand discharge in 
Fig. 2. 
If, however, we observe the right-hand discharges with a 
mirror revolving in the same direction as before, it is clear that 
the actual curvature of the discharge will be turned in the oppo- 
site direction (with reference to the motion of the mirror) to that 
TOVIVG 
Fic, 2. Fic. 4. 
in the case of the left-hand discharges. The consequence will 
be, that the appearance in the mirror, when the rate of revolu- 
tion is not too great, will be something like Fig. 7, instead of 
Fig. 6. As the speed of the mirror is increased, the convexity 
will diminish, and ultimately be replaced by a concavity of the 
same kind, although not so marked, as that in the case of the 
left-hand discharges. 
These diagrams show that each coil discharge commences with 
a bright spark passing directly between the terminals ; that this 
spark is, in general, followed by the pink light or are discharge, 
which passes first in the immediate neighbourhood of the mitial 
spark, and gradually extends like an elastic string in semicircular 
loops outwards ; and that the flame proper is a phenomenon 
attendant on the close of the entire discharge. It should be 
added that observations with a mirror revolving on a horizontal 
axis, and with a horizontal slit in front of the discharge, show 
that the disk is not simultaneously illuminated throughout, but 
that it isa locus of a curvilinear discharge which moves out- 
wards and expands in its dimensions from the centre. 
The mechanism of the discharge would therefore seem to be 
as follows :—In the first place, as soon as the tension is sufficient, 
the electricity from the terminals breaks through the intervening 
air, but with such rapidity that the fracture is like that of glass, 
or other rigid substance. This opens a path, along which, if 
there remains sufficient electricity of sufficient tension, the dis- 
charge will continue to flow. During such continuance the gas 
hecomes heated, and behaves like a conductor carrying a current, 
and upon this the magnet can act according to known laws. As 
sos595995+ 
N 
Fic. 6. 
long as the electricity continues to flow, the heat will at each 
moment determine the easiest, although not the shortest path for 
its subsequent passage. In this way the gas, which acts at one 
moment as the conductor of the discharge, and at the next as the 
path for it, will be carried further and further out, until the 
supply of the electricity from the coil fails, and the whole dis- 
charge ceases. Weare, in fact, led by these experiments to the 
conclusion that it is the gas in the act of carrying the current, and 
