556 POPULAR SCIENCE MONTHLY. 



magnetizing current is stopped, and soon after that the secondary 

 coil is unshortcircuited and connected to the galvanometer, and re- 

 mains in this condition during the remainder of the revolution. This 

 cycle of operations is repeated at every revolution. If then an electrical 

 oscillation is sent into the demagnetizing coils, and if it continues 

 longer than one revolution of the commutator, it will demagnetize the 

 iron core during that period of time in which the battery is discon- 

 nected and the galvanometer connected. The demagnetization of the 

 iron which ensues produces an electromotive force in the secondary 

 coil and causes a deflection of the galvanometer, and this deflection 

 will continue and remain steady if the oscillation persists. Moreover, 

 since this deflection is due to the passage through the galvanometer of 

 a rapid series of discharges, it is large when the oscillations continue 

 for a long time and are powerful, and small when they continue for 

 a short time or are weak. We can, therefore, with this arrangement, 

 receive on the galvanometer, just as on the mirror galvanometer used 

 in submarine cable work, a dot or dash, and, moreover, the magnitude 

 of these deflections is a measure of the energy of the wave. 



It is probable that when this arrangement is perfected it will 

 become exceedingly useful for making all kinds of tests and measure- 

 ments in connection with Hertzian telegraphy, even if it is not sensi- 

 tive enough to use as a long distance receiver. 



Of late years, a variety of wave-detecting devices have been brought 

 forward, which depend upon electrotysis. One of the best known of 

 these is that by De Forest and Smythe.* In this arrangement, a tube 

 contains two small electrodes like plugs, which may be made of tin. 

 silver or nickel, or other metal. The ends of these plugs are flat and 

 separated from each other by about one two-hundredth of an inch. 

 Sometimes the end of one of these plugs is made cup shaped and the 

 cup or recess is filled with a mass of peroxide of lead and glycerine. 

 In the interval between the electrodes is placed an electrolyzable mix- 

 ture, which consists of glycerine or vaseline mixed with water or 

 alcohol, and a small quantity of litharge and metallic filings. These 

 metallic filings act as secondary electrodes. When a small electro- 

 motive force is applied between the terminals of the electrodes of this 

 tube through a very high resistance of twenty or thirty thousand 

 ohms, an exceedingly small current passes through this mixture, and 

 it causes an electrolytic action which results in the production of chains 

 of metallic particles connecting the two electrodes together. If, in 

 addition to this, one terminal or electrode of the arrangement is con- 

 nected to an aerial wire and the other terminal to the earth, then on 

 the arrival of an electric wave creating oscillations in the wire; these 

 oscillations pass down into the electrolytic cell where they break up the 



* See U. S. A. Patent Specification, No. 716,000, Application of July 5, 1901. 



