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XLIX. Electrical Experiments with Mercury contained in 

 Tube By J. H. Vincent, M.A., D.Sc, A.R.C.So* 



[Plates VI. & VII.] 



Introduction. 



I~^IVE luminous effects are dealt with in the following 

 paper. They are : — 



(1) Persistent illumination in which a mercury arc 



increases and decreases in length rhythmically. 



(2) Periodic illumination in which the arc is intermittent. 



(3) Persistent illumination in which the arc is of constant 



length. 



(4) The emission of a faint hluish-green light from a space 



containing liquid mercmy and its vapour. 



(5) Phosphorescence of fused quartz. 



The first three of these are the chief matters to he 

 discussed, and for brevity they will be referred to by the 

 names (1) Necklace, (2) Fan, and (3) Ribbon effect respec- 

 tively. These names indicate the appearance observed on 

 viewing the arc in a revolving mirror whose axis of rotation 

 is parallel to the tube in which the arc occurs. The fourth 

 of the above effects will be called the Green Glow. 



After trying a variety of methods of mounting the tubes, 

 the plan of using WoulfFs bottles provided with side tubulures 

 near the bottom was tested. The tube was held horizontally 

 between two such bottles, the side tubulures of which were 

 fitted with rubber bungs, bored to receive the ends of the 

 tube. This proved so convenient that it has been adhered to 

 throughout most of the research. By connecting the spaces 

 above the mercury in the bottles together and with a 

 pressure- or exhaust-pump, the hydrostatic pressure of the 

 fluid in the tube could be varied at will and measured on an 

 open mercury manometer. The electric current was led to 

 and from the apparatus by clean iron wires piercing bungs 

 in the necks of the bottles and dipping into the mercury. 



The Necklace Effect. 



If a capillary tube of quartz glass be mounted in this 

 apparatus, it will generally be found possible, on filling the 

 tube and lower parts of the bottles with mercury, to form 

 the arc of rhythmically varying length in the tube. An 

 adjustable resistance is placed in series, and the circuit 



* Communicated by the Author. 



