IfiH PROF. TROFTON AND MR. NOBLE ON THE MECHANICAL FORCES ACTING 
To prever.t draugbts from Llo'wiiig down into the apparatus, a small glass hell jar 
eo^'ered the windlass, contact heing made by a wire passiDg through the small cork at 
the top. PQ is the small brass tnbe shielding the upper part of the suspen.sion. 
CD is the cylinder jjrotecting the point of snpjiort. AB indicates the position of tlie 
condenser inside the celluloid liall. Two cylindrical zinc vessels protect the 
apjiaratus ; these were earthed together with the ball containing the condenser. 
A plane mirror was attached to the condenser. This was viewed by means of a 
telescope and scale, through small mica windows in the zinc coverings. 
The potential was maintained by a Wimshurst machine, to the terminals of which 
was attached a Kelvin-AYliite voltmeter. 
■ ai 
n 
Fie:. 2 . 
Calculation of the Ejfect. 
Let ABfJD (fig. '!) he a given condenser charged to a surface density cr. Let it he 
movino- in the direction AB with velocitv u:. We have a 
^ - charge cr per unit ai'ea moving with velocity iv ; this is 
equivalent to a.current cric across unit length of the plane. 
The opposite charge on the other })late will produce an equal 
and opposite current ; therefore perpendicular to the })lane 
of the paper and parallel to that of the condenser we have a magnetic field equal to 
, mH' 
' Htt ’ 
the magnetic field ; tliis equals 2 /x 7 rcrhr'. 
The volume of the dielectric is Sc, where S is the total area of one armature of 
the condenser and e the thickness of the dielectric. The total magnetic energy is 
2iTu(Tav'^e. The total charge Q is , where Y is the potential difference l)etn-een 
^ " 477-e 
the nlates ; hence cr = The total magnetic energy is thus 27r/ru'-Sc 
^ n Aire ^ 
If N he the electrostatic energy of the condenser, N is ^ ( 4 ^^) ^ 
total magnetic energy is /xKNiC'—or, if r he the velocity of electric propagation. 
47 ro-/f’. The magnetic energy per unit volume is where H is the intensitv of 
tlie magnetic energv is N 
Sup})Ose now that the plane of the condenser makes an angle xjj with the direction 
of motion. Then magnetic energy of condenser is N( —) cos' xji. 
V 
Tlie total energy of the system is 
E = N 
1 + (') cos'- xjj 
