528 Dr. J. H. Vincent : Electrical Experiments 



Intermittent Electromotive Forces. 



If the secondary circuit of a small induction-coil is com- 

 pleted by means of a capillary tube containing mercury, and 

 mounted as usual, very interesting effects are obtained if the 

 middle of the tube is heated by some external source. If 

 the tube is short, and the pressure approximately that due to 

 the atmosphere, briliiant luminous effects are produced, which 

 are seen in the revolsing mirror to be of the necklace type, 

 but marked by broad bands of darkness corresponding to the 

 periods of little or no applied electromotive force. These 

 bands may be made very thin by using a large current in the 

 primary circuit. The outlines of the necklace at the ends of 

 the dark transverse bands are often marked by bright points 

 of light, which produce a most beautiful appearance when 

 the bands are broad. Using a quartz tube (10*7 cin., 6 mm., 

 *9 mm.) heated by a Bunsen flame, a brilliant intermittent 

 arc 4 cm. Icng could be maintained as long as desired. The 

 coil was a six-inch Apps coil with a two-volt accumulator on 

 the primary. 



Glass tubes can be used in this experiment, and will often 

 last an hour or longer. This is a very simple way of getting 

 mercury light for laboratory purposes. 



Under reduced pressure, or by the use of a long capillary 

 tube, the heat-supply serves to keep the column of mercury 

 broken, and, as with the ribbon effect already dealt with, the 

 mercury remains at rest in the tube. Using a small coil, 

 as before, it is possible in this way to get a very bright 

 mercury arc. In the revolving mirror the ribbon is seen to 

 be crossed by a series of dark lines, as before. 



Alternating Electromotive Forces. 



When alternating electromotive forces are employed, 

 effects of the fan type are always produced unless heat is 

 independently applied. Fig. 18 (PL VI.) is a photograph of 

 the arc in a quartz tube (4 cm., 4*2 mm., '55 mm.). The 

 electromotive force was 105 volts, and the frequency of the 

 supply Gl. The pressure was 80 cms. of mercury, and 

 the resistance in series with the lamp 150 ohms. The 

 frequency of the separate arcs is the same as the frequency 

 of the supply ; so that only half of each complete w r ave 

 of electromotive force is concerned in the production of the 

 luminosity. 



If a moving-coil ammeter is inserted in the circuit, the 

 current is found to be rectified ; but the selective effect 

 is variable, and the mean current changes in direction 



