Nikola Tesla: Colorado Springs Notes, 1899-1900 Image LV (55)

LV. This plate illustrates the discharge passing laterally across the laboratory from the tip of a wire, forming the “free” terminal of the extra coil, chiefly to the floor and top wire of the secondary. The sparks and streamers are very powerful and long, exhibiting many of the phenomena described in some cases before. One spark passing from the terminal of the extra coil across the laboratory is of rare beauty showing the individual discharges effected by the break wheel exceptionally well. A few sparks and streamers show luminous points on some places which may be “fireballs”. I have already dwelt at some length on this fascinating phenomenon and have explained what I consider to be its true nature. It may be of interest to state that such luminous points may be produced in other ways without partaking of the nature of the fireballs. One of these ways may be described here. It is well known - at least I assume so - that by causing two or more vibrations of different pitch to pass along a conductor, nodal points and points of maximum effect may be produced and caused to shift slowly along the conductor. Such a result I have frequently obtained with two vibrations of but slightly different period, the period of one of the vibrations being adjustable. If I recollect rightly I have described a result of this kind somewhere before this.

Substituting for a conductor of copper a vacuum tube of great length, I have also produced in the latter more or less luminous strips, striae or portions which would move along the tube with a velocity dependent on the relative wave length of both the vibrations,

and which was at will adjustable by an adjustment of the wave length of one of the vibrations. The truth is, however, that I observed this phenomenon in vacuum tubes long before without being able to render myself an account of the nature of the same until I obtained the same effect on a wire in the manner stated. Now it is quite clear that, since a streamer is a conducting path comparable to a wire, the same phenomenon may take place on the streamer itself - being the result of two (or possibly more) vibrations of different wave length. This is all the more probable as in such an apparatus two vibrations of this kind may readily occur since the capacity, or the inductance of the circuit or both, may undergo variations as the discharge is playing, thus modifying the period to a slight extent - sufficient to cause the production of this phenomenon. In fact I have frequently observed such variations of the constants of the circuit which is oscillating, these variations being produced in various different ways. Thus it may happen that there is seen on such a long spark or streamer a point or portion of greater or smaller luminosity, or more such points, moving along the path of the discharge with a small velocity, without having anything to do with the occurrence of the fireballs, as before explained.

If the interruption of a discharge in some part of its course, which was noted before, would not have been actually and unmistakably observed, I would think it quite possible that this phenomenon might be due to a dark part or nodal portion formed on the streamer in the manner above set forth, this part being either stationary or slowly shifting along the path of the discharge, as the case may be. I think that I shall be able to settle this point in the following experiment which I propose to carry out. The idea is to provide a streamer which will be preferably straight and will pass continuously through the same path, thus enabling an effect propagated along the streamer to be observed just as on a wire. The streamer should be preferably also of very great length. This I am convinced I can realize as follows:

A glass tube of pieces joined together temporarily, of rather large diameter, and of a length of, say, 30 to 50 feet is to be provided. The end of a well insulated wire, forming the “free” terminal of a coil, as the extra coil here used, is to be led in one end of the glass tube, in the center of the same so that the streamer, when formed on the point of this wire, will have the tendency to pass along the tube on the inside of the same. In order, however, to keep the discharge away from the glass, suction is to be applied on the other end of the tube or else a current of air is to be forced through the tube - from the side of the discharging wire towards the other open end of the former, in any convenient way - so as to compel the streamer to pass along the axis of the glass tube. If the tube is of large diameter I do not think that it will be difficult to carry out the experiment. Now this streamer is to be produced by oscillations of small wave length and under these conditions it will be quite easy to produce stationary or shifting, nodal or maximum points along the path of the streamer.

But, to return to the description of the present photograph, some of the paths described by the discharge are curious in the extreme. Many features dwelt upon before are again and even more clearly shown. So the “splashing”, the splitting up and reuniting is plainly visible. Some streamers strike the roof and one particularly was dangerous, the plate showing that after hitting the roof it divided in three parts following the structure. This will scarcely print. An ignition of the roof would have been unavoidable had the switch been held on only a fraction of a second longer. But in manipulating the switch I always took care to throw off the handle instantly when, by chance, one of the discharges

would dart to the roof. It is to be regretted that the building, although very large for ordinary experiments, did not allow the production of discharges still stronger than those before described, which would have been easily practicable with the present apparatus which - with more copper in the coils and particularly in the “extra coil”, and possibly without any change - would have in all probability enabled me to reach twice or three times the length of the actual discharges. In the experiment a great many sparks were seen to pas to the top turn of the secondary. These discharges would, without adequate provision, infallibly injure the condensers and the Westinghouse transformer and also other apparatus connected with the circuits or at a small distance from the same, no matter how well insulated they might be. By grounding the circuits in proper ways this danger is in a large measure reduced. In the experiment just described there were again 100 closures, rather short, effected by the switch and the other particulars not dwelt upon were the same as before.

This statement repeatedly made in the description of these plates should be specified. The truth is, each experiment required a special adjustment as the size and form of the terminals and the character of the discharge affected, to some extent, the constants of the oscillating system or systems. But the departures from the conditions designated as normal were very slight, the inductance of the primary or exciting circuit being varied only by inserting a very few turns of the regulating coil.