522 Dr. J. H. Vincent : Electrical Experiments 



fan effect had been looked upon as the consequence of the 

 contact and retreat of the two halves of the broken mercury 

 column. These observations show that whether or not this 

 is the case normally, it does not represent the only condi- 

 tions which will produce the fan effect. 



The readings for mean current and external resistance are 

 given in the table below, the last column of which gives the 

 effective resistance of the lamp calculated from the numbers 

 in the first two columns. The electromotive force was 

 100 volts throughout. 



Current. 



External 



Resistance. 



Effective 

 Resistance 

 of Lamp. 



•05 



in 



1890 



•14 



56 



659 



•18 



34 



518 



•25 



285 



372 



•30 



23 



310 



•32 



20 



295 



•35 



17 



269 



•40 



15 



235 



•45 



13 



208 



•48 



12 



196 



•55 



10 



172 



- -60 



9 



158 



•65 



8 



146 



•68 



7 



140 



•73 



6 



131 



•80 



5 



120 



The experiment w r as brought to a dose by the explosion 

 of! the tube. The effective resistance of the lamp rises 

 rapidly with increase in the external resistance. The drop 

 of volts in the lamp, calculated from the numbers in the first 

 two columns, varies only between 92'2 and 96, this latter 

 being the value for the greatest current, 



The Ribbon Effect. 



By increasing the frictional resistance to the motion of 

 the mercury in the tube, it is possible to obtain an arc of 

 constant length in mercury vapour under approximately 

 atmospheric pressure. 



This is the principle of a mercury-vapour lamp described 

 by Kent, Lacell, and the Silica Svndicate Limited (British 

 Patents Nos. 5596, 21834, 1908)/ This lamp consists of a 

 quartz tube initially filled with mercury. The column of 

 mercury is broken by a subsidiary heating coil in order to 

 start the lamp. Reservoirs of mercury at each end of the tube 



