We have no space to refer to the whole of the steps of discovery which led up to the invention of all the various forms of the modern electric kumascope or wave detector. Suffice it to say that the researches of Hertz in 1887 threw a flood of light upon many previously obscure phenomena, and enabled us to see that an electric spark, and especially an oscillatory spark, creates a disturbance in the ether, which has a resemblance in Nature to the expanding ripples produced by a stone hurled into water. Scientific investigation then returned with fresh interest to previously incomprehensible effects, and a new meaning was read into many old observations. Again and again it had been noticed that loose metallic contacts, loose aggregations of metallic filings or fragments, had a mysterious way of altering their conductivity under the action of electric sparks, lightning discharges and high electromotive forces. As far back as 1852, Mr. Varley had noticed that masses of powdered metals had a very small conductivity, which increased in a remarkable way during thunderstorms;[25] and in 1866, C. and S. A. Varley patented a device for protecting telegraphic instruments from lightning, which consisted of a small box of powdered carbon in which were buried two nearly touching metal points, and they stated that "powdered conducting matter offers a great resistance to a current of moderate tension, but offers but little resistance to currents of high tension."[26] We then pass over a long interval and find that the next published account of similar observations was due to Professor T. Calzecchi-Onesti, who described in an Italian journal, _Il Nuovo Cimento_ (see Vol. XVI., p. 58, and Vol. XVII., p. 38), in 1884 and 1885 his observations on the decrease in resistance of metal powders when the spark from an induction coil was sent through them.[27] These observations did not attract much attention until Professor E. Branly, of Paris, in 1890 and 1891, repeated them on an extended scale and with great variations, making the important observation that an electric spark _at a distance_ had a similar effect in increasing the conductivity of metallic powders.[28] Branly, however, noticed that in some cases of conductors in powder, such as the peroxide of lead or antimony, the effect of the spark was to cause a decrease of conductivity.