200 J. Trowbridge — Oscillations of Lightning discharges. 



the vacuum tube is held in one hand while the other hand 

 grasps one terminal of the transformer. In this case the 

 water resistance takes the place of the resistance of the air of 

 the room. The intensity of the discharge being thus much 

 diminished one can readily study various manifestations of 

 stratification which may perhaps be termed transitory stratifi- 

 cations in distinction to the stationary wave-like forms ob- 

 served in narrow tabes. The transitory stratifications can be 

 produced at will by touching suitable points of a vacuum tube 

 with the finger or by connecting such points with the ground. 

 Such stratifications are stationary as long as the ground con- 

 nection is maintained, and are independent of the rate of the 

 alternating machine which excites the transformer. It is evi- 

 dent that -the condenser action of the vacuum tube plays an 

 important part in this phenomenon. In observing the strise 

 and waving columnar form of the light excited in this man- 

 ner in tubes filled with rarefied gases, one is led to believe 

 that the stratified form of the Aurora Borealis is produced 

 in a similar manner. Fig. 3 is a form of the northern light 

 noticed by me. Let us suppose that a discharge of elec- 

 tricity takes place in rarefied air between A and B, and that 

 is a region of cloud or moisture. C can be regarded as the 

 finger or earth conductor which is applied to the tube of rare- 

 fied air and which seems to throw the discharge into transitory 

 stratification, and to give the waving form of the Northern 

 light. 



The pulsation, therefore, of the Aurora Borealis is in no 

 way, I believe, connected with any oscillatory discharge ; yet 

 certain writers have intimated that the glowing of vacuum 

 tubes which are connected with one terminal of a transformer 

 and the light of the Aurora is due to millions of electrical 

 oscillations per second. Now it is impossible to study the 

 question of the rate of oscillation of the brush discharge, by 

 means of Federsen's method for the light of the discharge is 

 not sufficient to produce a photograph. A brief consideration, 

 however, of the laws of electrical oscillations shows, I think, 

 that such writers- are mistaken. The rate of decay of the 

 amplitude of such oscillations is expressed by the well known 

 factor 



m 



2L 

 E 



Now in the case of a brush discharge R is enormously large. 

 A resistance of thirty or forty ohms was sufficient to com- 

 pletely damp the oscillations of the sparks studied by me in 

 the research on the damping produced on iron wires. In the 



