Electrical Experiments with Mercury contained in Tales. 507 



is completed by connexions with direct-current leads main- 

 tained at a potential difference of about a hundred volts. 

 The arc may be struck by (a) externally heating the middle 

 of the tube with a Bunsen flame or by an electrically-heated 

 wire wrapped round the tube, (b) sending a sufficiently 

 intense current through the mercury. The flickering inter- 

 mittent light at first produced is succeeded, on suitably 

 adjusting the resistance, by the arc of rhythmically varying 

 length, which under favourable conditions appears to the 

 unaided eye to be absolutely steady. On examining such an 

 arc in a revolving mirror, it appears to be drawn out into a 

 necklace-shaped band. A picture of such a band is shown 

 in fig. 1 (PL VI.), which is a reproduction of a photograph 

 taken on a moving photographic plate. 



Photographic Arrangements. — In order to take photographs 

 of the arcs on a plate moving with a known velocity, a camera 

 was fixed so as to point vertically downwards. The space 

 usually occupied by the dark slide was filled by a horizontal 

 board, upon which the mounted tube or, as we may call it, 

 the lamp was placed. The length of the tube was perpendi- 

 cular to the direction of motion of the photographic plate ; 

 the light from the lamp, after traversing a slit in the board, 

 passed downwards through the lens which focussed an image 

 of the tube on the plate. The plate was carried by a trolley, 

 running on the work-bench beneath the camera and actuated 

 by a falling weight. The trolley moved in a light-tight 

 tunnel of wooden planking, having a slit in it to allow the 

 light from the lamp to reach the plate. 



A Nernst glower placed behind a ring of holes in a rotating 

 metal disk provided a time-signal marking fractions of a 

 second on the moving photographic plate, on to which the 

 light was directed by a reflecting prism and a lens. 



The photograph (fig. 1) represents the necklace produced 

 in a quartz tube 4 cm. long, 4 2 mm. external diameter, and 

 •55 mm. internal diameter. The electromotive force in the 

 circuit was 98 volts ; the resistance in series with the lamp 

 was 120 ohms. Similar particulars will be given for short 

 as : 4 cm., 4*2 mm., *55 mm., 98 volts, 120 ohms. The 

 current was about a quarter of an ampere, and the volts 

 across the lamp 66. The hydrostatic pressure on the fluid 

 in the tube was 78 cm. of mercury. 



The line of lighter marks on the picture near the lower 

 edge are the time-signals, ^Jo or " a second apart, time 

 progressing from left to right. These marks may not be 

 clearly shown in the reproduction. Vertical distances on 

 the picture represent lengths parallel to the axis of the tube 



