90 
MESSRS. W. DE LA RUE AND H. W. MULLER ON THE 
When the revolving mirror is set in motion it is seen that the streamer-discharge is 
in reality to a great extent intermittent. At times a moderate rotation-velocity of 
the mirror serves to show this by the production of a number of distinct images, as 
seen in the left-hand drawing in fig. 19; at others it requires the full speed of the 
mirror. The discharge appears much more continuous with the point negative, so 
much so that the image is generally seen in the microscope as a sheet of light brightest 
near the point, and nearly uniform in a direction at right angles to the axis of the mirror ; 
the right hand drawing shows the appearance when the point is negative. The 
difference in the sound emitted when the point is positive and negative respectively 
appears to afford an additional proof of less continuity in the case of the positive, see 
also page 118. 
Fig 19. 
When a high resistance, 4 megohms for example, is inserted in the circuit, the 
character of the discharge is completely changed, instead of jumping across and forming 
an arc, a series of brilliant snapping sparks pass between the terminals at more or less 
rapid intervals, exactly like the sparks from a small Leyden jar ; these pierce a piece 
of writing paper interposed between the terminals producing minute holes. The spark 
does not jump at the full distance when the 4 megohms’ resistance is inserted ; and, 
to produce an almost continuous succession of intermittent sparks, it is usually 
necessary to approach the point to 0'30 inch, when, without resistance, the spark with 
8040 cells would jump and form the arc at 0\34 inch. 
When a point and a disc are used as terminals, and a piece of glazed writing 
paper, say 0‘00425 inch (0 '01079 centims.) thick, is laid on the disc, a very strong 
adhesion of the paper to the disc takes place, and it requires a strong pull to draw it 
along ; this phenomenon was ' first noticed by our assistant, Mr. Fram. In order to 
measure this force of adhesion we used a spring dynamometer, with a clip to hold the 
strip of paper, 1-| inch wide (the width of the disc). Under ordinary states of 
moisture of the atmosphere, with 8040 cells it required a pull — 
When the point was positive, 
grains, 
of 1300 
1300 
When it was negative, 
grains, 
of 2900 
2800 
1100 
3200 
Mean 1233 
Mean 2933 
