with Mercury contained in Tales. 529 



arbitrarily at intervals. If a few accumulators are placed 

 in the circuit, the change in direction still occurs ; but if 

 the cells are increased in number, a stage comes when the 

 current is always in such a direction as to discharge the 

 cells. 



Experiments in which the applied alternating electromotive 

 force was raised gradually to 1200 root mean square volts, 

 and the resistance varied on each voltage, were carried out. 

 No marked variation from the appearance as shown in fig. 18 

 was found on examining the arcs in the revolving mirror. 

 The fraction of a period during which the arc persisted was 

 slightly longer with higher voltages. 



On supplying heat to the tube, and using high alternating 

 electromotive forces, the appearance presented in the revolving 

 mirror depends on the pressure and length of tube. With 

 low pressures or long tubes, the heat produces a ribbon which 

 is lit up by the current. The ribbon is crossed by equidistant 

 dark narrow bands corresponding to the passage of the electro 

 motive force through its zero values. If the tube-length ard 

 pressure be such as that the heat supplied gives a necklace 

 motion to the column, then the appearance is that of a neck- 

 lace lamp, the band of light being again crossed by black 

 lines of no electromotive force. For these experiments a 

 small induction-coil used as a high-tension transformer was 

 found convenient. By heating the quartz tube in a Bunsen 

 flame, a brilliant arc from 5 to 10 cm. long can be maintained 

 as long as desired. 



Electrical Oscillations. 



The irregular vertical striae already noted in the photo- 

 graph of the ribbon (fig. 14, PI. VI.) are replaced by uniformly 

 spaced alternations of brightness when the lamp is run from 

 a dynamo (fig. 19). These bands are due to the change in the 

 electromotive force in the circuit consequent on the passage 

 of the teeth of the armature past the poles of the dynamo. 

 Similar effects are produced in the necklace pictures. In 

 fig. 20 the only circumstances changed from those of fig. 1 

 are that now the current is derived from a dynamo (103 volts) 

 and the resistance is 150 ohms. The alternations of bright- 

 ness have a frequency of 640 ; this number was found to be 

 nearly identical with the frequency of the replacement of one 

 tooth of the armature by the next, as calculated from the 

 speed of the dynamo determined as soon as possible after 

 the photograph was taken. 



Fig. 21 bears a strong likeness to fig. 19, but in this 

 figure the lamp was operated from cells and not from a 



