44 Proceedings of Royal Society of Edinburgh. [sess. 
and which has been rendered unresponsive to a second wave signal 
of the same intensity and frequency. 
Second . — In Marconi’s second instrument, the “varying or 
moving magnetic field,” upon which the invention is based, and 
by means of which the magnetic material is supposed to become- 
sensitive to high frequency oscillations, is entirely departed from. 
A continuous hand of iron or steel is passed over the poles of two 
horse-shoe magnets in the following order: — one south pole, one 
double north pole, and finally one south pole. Let the direction of 
motion be from left to right. The magnetic circuits of the two 
horse-shoe magnets are in part completed in the moving hand y 
and if its section he small relative to that of the electromagnets, 
the magnetic induction in the hand will depend largely upon its 
permeability. The two magnetic circuits, however, are in opposite 
directions ; and it is evident that if change of permeability affected 
the circuits equally and in the same sense, no E.M.F. would be 
produced in a coil symmetrically placed between them. 
The hand as it approaches the double north pole from the left 
is passing through a field increasing to a maximum ; and as it leaves 
on the right, it is passing through a field decreasing from a 
maximum. We have, however, seen (figs. 1 and 2) that under 
these conditions the superposition of oscillations produces in all 
cases an increase of induction (increased permeability) which is 
greater with increasing than with decreasing field. In other 
words, the increase of induction in the hand is greater immediately 
to the left of the double north pole than it is to the right. This 
differential action is further increased by the fact that the moving 
hand distorts the field in the direction of motion, viz., to the 
right. Hence the receipt of oscillations will he recorded in the 
telephone connected with the exploring coil symmetrically placed 
in reference to the magnets hut unsymmetrically placed in reference 
to field. 
It is further evident that once wireless telegraphic communi- 
cation has been established and is proceeding with reasonable 
constancy, the increase of induction which takes place in the 
increasing field (to the left) tends towards a maximum, and renders 
negligible any further induction change (increase or decrease) 
taking place in the decreasing field (to the right). 
