with Mercury contained in Tubes. 521 



soft-iron voltmeter read 9*5 volts across the lamp and 10*2 

 on open circuit. The frequency was 120. This tube also 

 gave the green glow on each side of the arc on heating with 

 a flame. 



By rapidly changing the number of cells in the circuit, it 

 was noticed that raising the volts lowered the pitch ; and it 

 was again found that raising the resistance external to the 

 lamp raises the pitch. A telephone receiver arranged as a 

 shunt on the lamp was used in these experiments. If 

 400,000 ohms are in series with the receiver a more musical 

 note is produced than with a lower resistance. 



Fan Effect not always due to Contact and Separation of 

 the Columns of Mercury. 



This will be a convenient place to record a very puzzling 

 observation which must be taken into account in forming 

 any theory of the fan effect. Using a quartz tube (10 cm., 

 4'o mm., *46 mm., 100 volts) the mean current and resistance 

 in the circuit were noted. The cenlral portion of the tube 

 was heated by the flame of a Bunsen burner which was kept 

 in the same position during the following experiments. A 

 frosted glow-lamp was placed behind the tube so that by 

 looking at the tube in the direction of the illuminated lamp 

 globe through a piece of flashed ruby glass, the position of 

 the m-ercury in the tube could be seen. 



The tube is occupied by the liquid mercury with the ex- 

 ception of a space two or three centimetres long over the 

 Bunsen flame. The part ot the tube thus chiefly occupied 

 by the mercury vapour was not of fixed length, but was 

 rapidly changing, and its ends on either side gave out the 

 green glow. The middle portion of the tube was not, how- 

 ever, quite free from liquid mercury, a few very small 

 particles being visible in the middle of the tube. As the 

 resistance external to the lamp decreased, it became impos- 

 sible to see these globules owing to the glare from the middle 

 portion of the tube which became brilliantly luminous as 

 the current increased. But on either side of the flashes of 

 light the tube appeared to be free from liquid mercury. 

 When the current was small, the middle portion contained a 

 number of microscopic scintillations. Unless a magnifying 

 glass be employed these liny sparks appear to occur in the 

 middle of a mass of vapour; the colour of these sparks looked 

 white in contrast to the green glow from the ends of the 

 vapour column. By the use of the revolving mirror it was 

 observed with surprise that the bright central light was of 

 the usual fan form. Up to this stage in the research the 



