June 26, T879] 



NATURE 



203 



primary to the secondary wire. It was ultimately entirely 

 remade in February, 1878, and the secondary wire coiled 

 on a separate ebonite cylinder to insure efficient insula- 

 tion, which was accomplished. 



In every case where the strata are to the eye or rotating 

 mirror perfectly steady, slight deflections of the needle 

 are seen ; these generally indicate a resultant direct cur- 

 rent (break-contact), and in the fewer number of cases 

 an inverse current indicating, in the first case, a sudden 

 decrease and slow increase of current through the tube. 

 These deflections, though very manifest, do not amount 

 to more than about three or four divisions of the galvano- 

 meter scale, a deflection which indicates a current of only 

 o'ooooooooo23 W. At the advent or retreat of a stratum 

 at the positive pole there is frequently produced a devia- 

 tion of 300 divisions, indicating a current of O'oooooooi8i2 

 W ; before a stratum leaves the positive terminal or dies 

 out on it, there is usually a tremulous motion of that 

 stratum visible to the eye and indicated by rapid pulsa- 

 tions of the galvanometer. 



On the suggestion of Prof. Clerk Maxwell we have re- 

 cently introduced the telephone into the primary current, 

 as shown in Fig. 20, and also in 

 the secondary current of coil 819. 



In all cases where the con- 

 denser C was discharging itself 

 gradually through the tube, a 

 low rustling sound was distinctly 

 audible to sensitive ears so long 

 as the stratification remained 

 appare7itly perfectly steady. 

 When the phase of confused 

 stratification which immediately 

 precedes extinction was reached, 

 the sound in the telephone be- 

 came very loud and rose in 

 pitch, with some tubes becoming 

 quite shrill. These results, there- 

 fore, confirm the conclusion 

 already arrived at from other 

 experiments, namely, that the 

 discharge in vacuum tubes is 

 intermittent ; but we do not 

 pretend that they make it mani- 

 fest that stratification is de- 

 pendent upon intermittence. 



In the course of our experi- 

 ments, using sometimes 1 1,000 

 cells, we have arrived at the 

 following facts : — 



1 . The discharge in a vacuum tube does not differ essen- 

 tially from that in air and other gases at ordinary at^no- 

 spheric pressures j it cannot be considered as a ctirrent in 

 the ordinary acceptation of the term, but must be of the 



, nature of a disruptive discharge, the molecules of the gas 

 acting as carrie7-s of electrification. The gases in all 

 probability receive impulses in two directions at right 

 angles to each other, that from the negative being the 

 more continuous of the two. Metal is frequently car- 

 ried from the terminals and is deposited on the inside of 

 the tube, so as to leave a permanent record of the spaces 

 between the strata. 



2. As the exhaustion proceeds, the potential necessary 

 to cause a current to pass diminishes up to a certain 

 point, whence it again increases, and the strata thicken 

 and diminish i?i number, until a point is reached at 

 which, notwithstaiiding the high electromotive force 

 available, no discharge through the residual gas can 



detected. Thus, when one pole of a battery of 8,040 

 lIIs was led to one of the terminals of tube 143, Fig. 21, 

 which has a radiometer attached to it, the other terminal 

 of the tube, distant only o"i inch, being connected 

 through a sensitive Thomson galvanometer to the other 

 pole of the battery (eaith), the current observed was not 



greater than that which was found to be due to conduc- 

 tion over and through the glass. Although no current 

 passed, the leading wires acting inductively stopped the 

 motion of the radiometer, as has been observed by Sir 

 William Grove. 



3. All strata have their origin at the positive pole. 

 Thus, in a given tube, with a certain gas, there is pro- 

 duced at a certain pressure, in the first instance, only 

 one luminosity which forms on the positive terminal, 

 then, as the exhaustion is gradually carried further, it 

 detaches itself, moving towards the negative, and being 

 followed by other luminosities, which gradually increase 

 in ntimber up to a certain point. 



4. With the same potential the phenomena vary irre- 

 gularly with the amount of current. Sometimes, as the 

 current is increased, the number of strata in certain tubes 

 increases, and as it is diminished their number decreases \. 

 but with other tubes the number of strata frequently in- 

 creases with a diminution of current. If the source of 

 the current is a charged condenser, the flow being from 

 one of its plates through resistances and the tube to the 

 other ; then, as the potential of the condenser falls and 



FlG. 20 



the current diminishes, the number of strata alters ; if 

 the strata diminish in number with the fall of potential, 

 then the stratum nearest the positive wire disappears on 

 it, the next then follows and disappears, and so on with 

 others ; if, on the other hand, the charge of the condenser 

 is very gradually increased, the strata pour in, one after 

 the other, in the most steady and beautiful manner from 

 the positive. 



Fig. 21. 



5. A change of current frequently produces an entire 

 change in the colour of the strata : for example, in a 

 hydrogen tube from a cobalt blue to a pink. It also 

 changes the spectrum of the strata ; moreover, the spectra- 

 of the illuminated terminals and the strata differ. 



