224 PROCEEDINGS OP THE AMERICAN ACADEMY. 



restored. If, on the other hand, we use a brass rod and an iron tube, 

 when the rod is put in without the tube the discharge is bright. If 

 we slip the iron tube over the rod, the discharge stops." * 



In a paper upon the " Absorption Power of Metals for the Energy 

 of Electric Waves," t Bjerknes has given some results which show 

 the great damping power of magnetic metals upon electric oscillations 

 of very high frequency (100,000,000 double oscillations per second) . 

 The apparatus used was, in a slightly modified form, the Hertz vibra- 

 tor and circular resonator, but in place of the spark micrometer in 

 the resonating circuit he used a much more exact and sensitive arrange- 

 ment, — a kind of quadrant electrometer with two quadrants to which 

 the ends of the resonating circuit were directly joined. He employed 

 among others resonators of copper, iron, and nickel identical in size and 

 construction. The length of wire in each case was 123 cm. and the 

 diameter 0.5 mm. The length of wire joining the plates of the Hertz 

 vibrator could be varied at pleasure. By varying this, the length of 

 wire necessary for best resonance was found in each case, and the 

 electrometer throws were observed for five different lengths of the 

 primary circuit, including the one for best resonance effects. The 

 graphic representation of these results shows plainly that the metals 

 differ greatly in their power of damping electric oscillation. The 

 electrometer throws were much smaller for the iron and nickel than for 

 the copper, and the curves for iron and nickel come less sharply to a 

 maximum. He further conclusively shows that the damping power 

 of the metals experimented upon increases with their resistance and 

 magnetic susceptibility, and concludes that the magnetic properties of 

 iron and nickel are called into play by their extremely rapid alternations 

 of the magnetizing forces. He notes the fact that the maxima for iron 

 and nickel seem somewhat displaced to one side, which may indicate a 

 greater period, but says that such a displacement of the maxima enters 

 iu case of greater damping, so that best resonance does not corre- 

 spond to exactly equal periods of the two circuits, and adds that a 

 quantitative investigation is necessary to determine to which of the 

 two causes the effect may be referred. 



From this brief survey of the field, it is seen how, with more exact 

 and refined means of measurement, some of the results expected, when 

 magnetic metals replaced copper in circuits through which rapid 

 electric discharges were taking place, have finally been observed. 



* Pliil. Mag. (5.), XXXTI. 456. July, 1891. 

 t Poggendorff, Annalen, XL VII. 69. 1892. 



