Every experimenter in electricity who has had to do with Geissler tubes has at one time or another marveled at the beauty and the mystery of the phenomena of stratification. In producing the stratified light in vacuo the Ruhmkorff coil has been generally regarded as the only practically available means for the illumination of the vacuum. So common has been this impression that few experimenters, not even the makers themselves of the tubes, have seemed aware of the valuable possibilities of the influence machine for this particular form of experiment. This, of course, has not been without its reason. While no special precaution or nicety of manipulation is required in exhibiting' stratified tubes with the coil, when such a tube is essayed to be used with the static machine the first experiences are usually disappointing As the vacuous space merely lights up with the familiar unbroken glow of the ordinary Geissler tube, without any traces of striation, the experimenter naturally concludes the desired effect to be impossible of attainment with the static machine. However, with attention to a few simple details of technique, striations can be developed with the influence machine, having a distinctness and beauty, uniformity, and fixity of position never realized with the coil method of excitation.

In bringing out the striae with the static machine two vital conditions are to be observed; namely, sufficient, though not excess ive current through the tube; and the careful avoidance of all sparking, even of the minutest character, at possible imperfect contacts in any part of the circuit outside the tube. The first implies a generator of sufficient size, to begin with, whose output can then be diminished or increased by regulation of its speed. In exciting stratification tubes with machines of the Holtz type it is generally only necessary to connect their terminals direct to the poles of the machine, with careful attention to perfect metallic contacts; the minutest break where disruptive sparking can occur destroys the striation and diffuses the light. This precaution attended to, the proper strength of current must be found by experimental regulation of the speed of the generator. As the machine slowly starts, the light within the tube is first seen as a thick nebulous line along the axis of the tube between the elec-trodes. This, at first continuous and steady, soon shows signs of uneasiness as the machine speeds up, and presently wavers and breaks into a beautiful series of brilliant, evenly spaced, isolated bands or strata which, when the current strength attains a certain value, settle into fixed positions and remain perfectly motionless. It is significant of this feature of steadiness that it is one specially remarked by De la Rue as characteristic of the striae developed in his vacuum tubes by direct galvanic currents, during his now historical experiments with high-potential batteries of many hundred cells; thus, again, suggestion of the probable ultimate identity of the natures of static and voltaic forms of electrical action is here vividly brought to mind. In the study of striation by the present method, the almost total absence of the violent oscillatory movements, frequent blurring and overlapping, and uneven spacing of the striae so characteristic of coil excitation, is an obvious advantage. The remarkable constancy of the stratification renders easy the making of photographic studies, with time expi -tires. Fig. 204 illustrates the beautiful uniformity, even spacing and perfect segregation of the striae produced with a Wimshurst machine in a 12-inch tube, the effect being photographed with a three-minute exposure. In exciting stratification with the Wimshurst machine (which has probably been more largely made and used by amateurs than any other type), a simple device which from its function might be termed an atmospheric rheostat is required in conjunction with the tube. This necessity arises from a well-known peculiarity of Wimshurst machines, especially those of the sectorless type. If such machines are attempted to be run on closed circuit, or on a circuit having too little resistance, such, for example, as might be offered by a single Geissler's tube, their fields suffer such a diminution of potential as to cause a serious falling off of the output of the generator; thus, a tube may not receive sufficient energy even from a large generator to establish the stratification. This is obviated by supplementing the resistance of the tube by that of two air gaps, one on each side of the tube in series with it. But as these gaps must not be spark gaps, the construction shown in Fig. 205 is adopted. Two smooth metallic disks three or four inches in diameter machine with well rounded edges are mounted in vertical positions on short insulating standards. Opposite the center of each disk and facing it is an insulated sliding rod terminating in a fine sharp point capable of longitudinal adjustment through a space of two or three inches.. The tube to be exhibited is connected as shown between the two middle posts; the two outer posts are in unbroken metallic connection with the opposite poles of the generator. With this arrangement the current passes the air gaps between the points and disks as a silent, continuous, non-sparking discharge and the stratifications within the tube are beautifully developed. In using the device, proper polarity is of vital importance. The point at the left must be connected to the negative side of the generator - that side which shows the brush effect on the collecting combs; the disk at the right is wired to the positive pule. If this order be reversed, sparking occurs at the resistance gaps and the effect is destroyed.

Fig. 204 - Striatum produced with influence machine.

The "stratified" tube, without which no collection of vacuum tubes is complete, is a specially prepared Geissler tube exhausted to just the proper degree, and containing some particular residual gas or vapor whose molecular movement has been found especially compliant to certain conditions of vibration, in the peculiar ordering of which the phenomena of "resonance" has had strong suggestion of probable participation.

Fig. 205 - Atmospheric rheostat for stratification in vacuo with Wimshurst.