548 



NATURE 



\_Sept. 19, 1878 



caused to pass through a vacuum tube to illuminate it 

 strongly. 



"The discharge from the point, when positive, has 

 mainly to the naked eye a stream-like appearance, con- 

 sisting of a waving line of light surrounded by a very 

 faint sugar-loaf luminosity, and producing a rattling or 

 strong hissing sound ; from the point when negative it 

 consists of a glow of light paraboloidal in form, and 

 extending from the point to the disc, but much more 

 brilliant at the point, the noise of the discharge being 

 much less than when the point is positive. The disc, 

 especially when positive, soon becomes covered with a 

 peach-like bloom, and the deposit assumes the appear- 

 ance of Newton's rings. 



" Small as this current is, it is, nevertheless, very mani- 

 fest from its brightness and the rattling, or loud hissing 

 noise it produces; it frequently continues for some 

 minutes before the spark actually jumps between termi- 

 nals placed at the striking distance asunder ; the streamer- 

 discharge becomes brighter just before the jump and 

 formation of the arc, and this suggested the possibility of 

 particles being carried off in increasing quantity from the 

 point to the disc, and thus contributing to the production 

 of the spark. In order to test this hypothesis, the termi- 

 nals were placed at the striking distance, and a con- 



tinuous blast from a blowpipe bellows sent between them ; 

 this did not, however, have any effect on the length of 

 the spark, but it deflected the arc when once it had 

 formed. 



" Under the microscope the discharge, from the point 

 when positive, is seen to consist of several streams of 

 light, which twist round each other like loosely-bound 

 strands, as shown in Fig. 8, representing the discharge 

 between the terminals, a point and a disc, in a horizontal 

 discharger. Part of the discharge from the point nega- 

 tive is shown in Fig. 9. Fig. 10 shows on a scale of 4 

 to I the phenomenon obtained with 11,000 cells, the 

 streamers and glow proceeding simultaneously from the 

 opposite points of a vertical discharger and existing inde- 

 pendently of each other." 



In order to study these discharges the authors con- 

 structed a microscope (Fig. 11) with a revolving mirror 

 placed at an angle formed by the two tubes composing 

 the body of the microscope, which is bent for the con- 

 venience of observation. That part beyond the upright 

 is formed of ebonite, in order to protect the eye from 

 accidental shocks. The discharger used when this 

 microscope was employed was horizontal, as seen in the 

 figure, in order to permit of the employment of the 

 mirror which was mounted on a horizontal axis ; the 



Fig. II. — Microscope with Revolving Mirror. 



point P was placed in the fixed upright, and the disc D 

 in that upright which is attached to the adjustable slide. 

 The screw S' is used to approach or recede the micro- 

 scope in order to adjust to focus ; s" to bring the point 

 horizontally, and S to elevate or depress the microscope 

 by means of a pinion and rack to adjust the point ver- 

 tically in the field, o is the object-glass and E the eye- 

 piece ; w a multiplying-wheel giving five revolutions of 

 the mirror for one turn of the handle, which may be 

 rotated with ease 200 times in a minute, so that the 

 mirror can be caused to make 1,000 revolutions in a 

 minute, or about seventeen revolutions in a second. The 

 whole framework of the stand is of ebonite, to ensure 

 insulation. 



"When the revolving mirror was set in motion it was 

 seen that the streamer-discharge was in reality, to a 

 great extent, intermittent. At times a moderate rotation- 

 velocity of the mirror served to show this by the produc- 

 tion of a number of distinct images, as seen in the left- 

 hand drawing in Fig. 12; at others it required the full 

 speed of the mirror. The discharge appeared much 

 more continuous with the point negative : so much so 

 that the image was generally seen in the microscope as a 

 sheet of light brightest near the point, and nearly uni- 

 form in a direction at right angles to the axis of the 

 mirror. The right-hand drawing shows the appearance 

 when the point is negative. The difference in the sound 

 emitted when the point is positive and negative respec- 



FiG. 12.— Analysis of Streamers and G!ow. 



tively appears to afford an additional proof of less con- 

 tinuity in the case of the positive. 



" When a high resistance — four megohms, for example 

 — was inserted in the circuit, the character of the dis- 

 charge was completely changed. Instead of jumping 

 across and forming an arc, a series of brilliant snapping 

 sparks pass between the terminals at more or less rapid 

 intervals, exactly like the sparks from a small Leyden 

 jar ; these pierce a piece of writing paper interposed 

 between the terminals, producing minute holes. The 

 spark does not jump at the full distance when the four 

 megohms resistance is inserted; and, to produce an 

 almost continuous succession of intermittent sparks, it 

 was usually necessary to approach the point to 0*30 inch, 

 when, without resistance, the spark with 8,040 cells 

 would jump and form the arc at o"34 inch." 



When charcoal terminals were used the jump of the 

 spark was about the same as with other terminals having 

 a similar shape ; the charcoal points could be separated 

 when 8,040 cells were used to 1-25 inch generally, and 

 occasionally to r 5 inch, without breaking the arc. The 

 arc presented the ordinary characters, as shown in Fig. 

 13; but that which takes place when the terminals are 

 vertically one over the other is different, on account of 

 its being undisturbed by upward air currents which 

 deflect the arc. 



The length of the spark differs in various gases at 

 ordinary atmospheric pressures, but the authors found 



