Another line of experiment, which has been assiduously followed, was to induce by electro-dynamic induction a current or luminous discharge in an exhausted tube or bulb.  This matter has received such able treatment at the hands of Prof.  J.J.  Thomson that I could add but little to what he has made known, even had I made it the special subject of this lecture.  Still, since experiences in this line have gradually led me to the present views and results, a few words must be devoted here to this subject.

It has occurred, no doubt, to many that as a vacuum tube is made longer the electromotive force per unit length of the tube, necessary to pass a luminous discharge through the latter, gets continually smaller; therefore, if the exhausted tube be made long enough, even with low frequencies a luminous discharge could be induced in such a tube closed upon itself.  Such a tube might be placed around a ball or on a ceiling, and at once a simple appliance capable of giving considerable light would be obtained.  But this would be an appliance hard to manufacture and extremely unmanageable.  It would not do to make the tube up of small lengths, because there would be with ordinary frequencies considerable loss in the coatings, and besides, if coatings were used, it would be better to supply the current directly to the tube by connecting the coatings to a transformer.  But even if all objections of such nature were removed, still, with low frequencies the light conversion itself would be inefficient, as I have before stated.  In using extremely high frequencies the length of the secondary—­in other words, the size of the vessel—­can be reduced as far as desired, and the efficiency of the light conversion is increased, provided that means are invented for efficiently obtaining such high frequencies.  Thus one is led, from theoretical and practical considerations, to the use of high frequencies, and this means high electromotive forces and small currents in the primary.  When he works with condenser charges—­and they are the only means up to the present known for reaching these extreme frequencies—­he gets to electromotive forces of several thousands of volts per turn of the primary.  He cannot multiply the electro-dynamic inductive effect by taking more turns in the primary, for he arrives at the conclusion that the best way is to work with one single turn—­though he must sometimes depart from this rule—­and he must get along with whatever inductive effect he can obtain with one turn.  But before he has long experimented with the extreme frequencies required to set up in a small bulb an electromotive force of several thousands of volts he realizes the great importance of electrostatic effects, and these effects grow relatively to the electro-dynamic in significance as the frequency is increased.

Now, if anything is desirable in this case, it is to increase the frequency, and this would make it still worse for the electro-dynamic effects.  On the other hand, it is easy to exalt the electrostatic action as far as one likes by taking more turns on the secondary, or combining self-induction and capacity to raise the potential.  It should also be remembered that, in reducing the current to the smallest value and increasing the potential, the electric impulses of high frequency can be more easily transmitted through a conductor.