228 
MESSRS. W. DE LA RUE AND H. W. MULLER ON THE 
the other plate of the condenser, and through the safety wire to the other terminal T 
of the tube. The secondary wire of coil 819 is connected to a delicate Thomson 
galvanometer T. 
The apparatus being so arranged, if K is pressed down the condenser charges up 
(more rapidly when K' is also pressed down than when the current is allowed to 
pass through the resistance FR"), and when it has reached a certain potential the 
current commences to pass steadily through the tube and the primary of the induction 
coil 819, and will continue to do so after the connexion with the battery is interrupted 
by raising the key K. 
The condenser discharges itself more or less rapidly through the tube according to 
its resistance, and the external resistance introduced into the circuit at FR, FR', and 
the megohm. The fluid resistance FR" has no effect upon the time occupied by the 
charge running out of the condenser, as the current does not pass through it when 
the key K is allowed to rise so as to disconnect the battery, as will be easily seen by 
referring to the diagram. The only use of FR" is to regulate the rate of inflow of 
the charge, and it may be so adjusted as to feed the condenser exactly as fast as it 
loses its charge. By pressing down the keys K' and K the condenser is maintained 
at the highest potential possible with a given number of elements, a given tube, and 
a certain external resistance. 
It is quite evident that even if pulsations do take place in the current through the 
tube, no effect would be produced on the galvanometer in connexion with the 
secondary of the induction coil, provided the rise and fall of the current were equal 
and in equal periods. The case would, however, be different provided either the rise 
or the fall were more rapid relatively to the other, and one might expect under these 
circumstances that there would be some movement of the needle of the galvanometer, 
notwithstanding that its period of oscillation was not synchronous with the pulsations 
of the current. 
A dynamometer would evidently be a much better instrument for detecting 
pulsations in the primary current, because the reversal of the current does not affect 
its deflection, which is cumulative in the same direction. We had one constructed as 
delicate as possible in which the moveable coil was suspended by a platinum wire 
only O'OOl inch in diameter, but it was found to be far too sluggish for our purpose. 
Between the terminals of the Thomson galvanometer is inserted a shunt-box, by 
which the current of the secondary wire may be reduced to x^th, y^th, or - x oVo th 
part before passing through it ; this shunt-box also has a short-circuit plug, by means 
of which, when so desired, the current may be entirely shut off from the galvanometer. 
The shunt-box is not shown in the diagram. 
In order to ascertain the direction of the current in the secondary wire, the 
condenser was disconnected in the first instance, the short-circuit plug of the gal- 
vanometer being removed, and contact made with the battery so as to send the 
current through the tube, the swing of the galvanometer indicated the direction, 
