703 



wore a very light mirror iDade from a silvered microscope covering 

 glass. The light of a single wire lamp was thrown on the mirror 

 through the space between the two coils. The reflected ravs formed 

 an image on a scale placed at a distance of 45 cm When the 

 cylinder was set vibrating this image was broadened into a band, 

 the width of which determined the double deviation. 



In order to obtain resonance, it must of course be possible to 

 regulate the length of the glass wire. For this purpose we used a 

 clamping arrangement by which the glass wire could be tightly 

 held at different points of its length. 



The clamp and the suspending wire with the cylinder could rotate 

 together about a vertical axis in a fixed column. The effective current 

 was read on a precision instrument. Finally, the whole apparatus 

 was surrounded by an arrangement by which the terrestrial magnetic 

 field could be compensated. We shall i-evert to it further on. 



§ 5. The iwperiinents. 



Let us now examine the principal disturbing causes. 



1. At the ends of the cylinder alternating poles are induced. 

 Acting on these the horizontal component of the terrestrial field can 

 give rise to a couple alternating with the same frequency as the 

 current and tending to rotate the cylinder about a horizontal 

 axis. (Effect I) 



Rotations of this kind have not, however, been observed by us. 



2. According to the views of Wkiss the ferromagnetic crystals 

 are lying irregularly in all directions. It maj tiierefore happen 

 that some of them are directed in such a way that their magnetism 

 is not reversed by the alternating field. In this case there will be 

 a permanent horizontal component of the magnetisation, which, 

 acted upon by the alternating horizontal component of the magnetic 

 field in the coil, will give rise to an alternating couple around the 

 vertical axis with the same frequency and phase as the alternating 

 field (Effect II). 



3. The axis about which the cylinder rotates will not coincide 

 accurately with its magnetic axis. 



A permanent horizontal magnetic force such as that of terrestrial 

 magnetism, will therefore produce torsional oscillations of the cylinder. 

 The couple which excites these oscillations has the same phase as 

 the magnetisation and ^in the case of strong currents) as the alter- 

 nating current itself. 



4. It is easily seen that the P'oucault currents which are induced 



