330 
PROF. H. M. MACDONALD ON THE EFFECT PRODUCED BY 
A Perfectly Absorbing Obstacle. 
When the ol)stacle is perfectly absorbing, the conditions to be satisfied at its surface 
are that the electric and magnetic forces in the incident waves are annihilated rhere. 
This requires an electric current distribution on tbe surface which will produce the 
macrnetic force tangential to the surface of the obstacle in tbe incident waves and a 
magnetic current distribution on the surface which will produce the electric force 
tangential to the surface of the obstacle in the incident waves. As in the former case, 
it is sufficient to discuss the case of a Hertzian oscillator at a point outside the 
obstacle. Taking the origin of co-ordinates at the oscillator and the axis of z 
coincident with the axis of the oscillator, the components of the electric and magnetic 
forces at a point p, { due to the oscillator are given by 
where 
and therefore the compoitents of the electric current and magnetic current distri¬ 
butions to be assumed on the portion of the surface of the obstacle on which waves 
are incident are given by 
[my' — 71/3'), [no! — ly), (//3' —ma'); 
(mZ'-nY'), {7iX!-lZ'), (ZY'-mX'); 
where Z, 7n, n are the direction cosines of the normal to the surface, these are 
equivalent to 
where only the principal parts have been retained ; writing 
ii/'f'i — Z'-i, ~ P'1’ 
these become 
— lkWi —LKhijx-^ e‘''^^’*~’‘'yriV, lk^ {lY + mgi) e''‘’^'^^~'''^, 7\, 
— LK^ {m lY {wi^iY + ^V + Pi^)} e'-'’^^*~’'''’f7\, 
tK“ [Igi — mY) nj 
