Coupled Circuits used in Wireless Telegraphy. 577 



rods of wood, one near each end, and both perpendicular to 

 the axis of the beam. In each of these rods two holes, equi- 

 distant from the beam, are drilled, through which the upper 

 ends of the thread forming the V suspension of each 

 pendulum pass. Ordinary binding screws on the upper 

 surface of the rods serve to make fast the strings and render 

 quick adjustment of length easy. The distance between the 

 holes in each rod should, of course, be sufficient to enable 

 the V of string supporting each bob to clear the bed and its 

 supports, the bobs of the pendulums being vertically under 

 the beam and bed, and able to move only in a vertical plane 

 parallel to the axis of the beam. 



To the upper side of the beam opposite the runners at 

 either end are attached two platforms, on which the masses 

 used to vary the coupling can be placed. It was shown in 

 § 3 that the square of the coupling or sin 2 i|r is equal to 



m ] m 2 



(M + mi)(M + m 3 ) 



where M is the total mass of the beam, so by increasing the 

 load on the beam, M in the above will be increased and the 

 coupling diminished. 



Equal masses should be placed on each platform, and the 

 platforms should be directly over the bearing spindles when 

 the system is at rest, in order to reduce to a minimum any 

 tendency the loads might have to bend the beam. 



8. In order to exemplify the use of the model for the purpose 

 of illustrating the properties of coupled circuits, I will 

 describe some experiments on the relation of the resultant 

 periods to the natural periods and the coupling. 



Adjust the pendulums to be of about the same length, say 

 2 ft. 6 in. or 3 ft., put different masses for the bobs, say 

 make one twice as heavy as the other, and make the coupling 

 loose by arranging that the mass of the beam shall be at least 

 ten times the mean of the masses of the bobs. 



Determine by observation the natural period of each 

 pendulum. As has already been explained, this is done for 

 the first by placing the bob of the second on its platform and 

 counting the number of swings made by the first in a given 

 time in the usual way : similarly, determine the natural 

 period of the second pendulum. The natural periods can 

 now be computed by means of the formulae in § 3 if the 

 masses and lengths have been measured. 



To the actual mass of the beam and its load must be added 

 a correction for the wheels to obtain the value of M in the 



Phil. Mag. S. 6. Vol. 25. No. 148. April 1913. 2 R 



