is oscillatory, and the electric strain in the non-conductor, or dielectric, viz. the air around the plates, dies away by a series of rapidly alternating electric strains in opposite directions. Now, at this point I must recall to your recollection that, in speaking about the production of air waves, I pointed out that one condition essential to the production of an air wave was that there must be a very sudden application or release of the air-pressure, such as is caused by an explosion or escape of compressed air. We cannot produce an air wave by moving any object such as a fan slowly to and fro through the air. In order to produce an air wave we must strike the air a very sudden blow, or, which comes to the same thing, we must apply and remove a very sudden pressure to the air; and under these circumstances we start into existence an air wave, which travels away from the vibrating or rapidly moving body, and continues its journey out into surrounding space. I want to show you that, in the case of the Hertz oscillator, these very sudden reversals of electric strain in the air or space round about it, which take place at the moment when the oscillatory spark passes between the knobs, creates in a similar manner what is called an _electric wave_, which travels out into the space around. The point you must appreciate is, that just as an air wave conveys away to distant places a rapidly alternating compression made in the air by a vibrating body at a particular place, so an electric wave conveys away to distant places an alternating electric strain, which is originated at some point in the medium by the oscillatory discharge of some form of condenser. Before, however, we can demonstrate this fact, we must have some means for detecting the influence of what we call an electric wave. You will remember that, in the case of experiments with air waves, I used a sensitive flame in order to make evident to you the presence of waves in the air which you could not see, so here I must use an appropriate detector for electric waves, the operation of which will render evident to us the existence in the space round our electric oscillator of the electric waves we cannot see. Time will not permit me to discuss all the different forms of electric-wave detector which have been invented. For our present purposes we must limit ourselves to the description of one plan, which depends on the remarkable fact that finely powdered dry metal or metallic filings are non-conductors of the electric current until they are subjected to an electromotive force exceeding a certain value, when the metallic filings at once pass into a conductive condition.