of Electric Waves through Wires. 121 



if the wire lies along the middle of the strip, and much stronger 

 when near its edge. Just as in the case of distribution under 

 electrostatic influence the electricity would prefer to collect on 

 the sharp edge of the strip, so also here the current tends to 

 move along the edge. Here, as there, it may be said that the 

 outermost parts screen the interior from outside influence. 



The following experiments are somewhat neater and equally 

 convincing. I inserted into the conductor transmitting the 

 waves a very thick copper wire, 1*5 metre long, whose ends 

 carried two circular metallic disks of 15 cm. diameter. The 

 wire passed through the centres of the disks ; the planes of 

 the disks were at right angles to the wire ; each of them had 

 on its rim 24 holes, at equal distances apart. A spark-gap 

 was inserted in the wire. When the waves traversed the wire 

 they gave rise to sparks as much as 6 mm. long. A thin 

 copper wire was then stretched across between two corre- 

 sponding holes of the disks. On doing this, the length of the 

 sparks sank to 3*2 mm. There was no further alteration if a 

 thick copper wire was put in the place of the thin one, or if, 

 instead of the single thin wire, twenty-four of them were 

 taken, provided they were placed near each other through the 

 same two holes. But it was otherwise if the wires were dis- 

 tributed over the rim of the disks. If a second wire w r as 

 inserted opposite the first one, the spark-length fell to 1*2 mm. 

 When two more wires were added midway between the first 

 two, the length of the spark sank to 0*5 mm.; the insertion of 

 four more wires still in the mean positions left sparks of scarcely 

 0*1 mm. long ; and after inserting all the twenty-four wires at 

 equal distances apart, not a trace of sparking was perceptible 

 in the interior. The resistance of the inner wire was never- 

 theless much smaller than that of all the outside wires taken 

 together ; we have also a still further proof that the effect does 

 not depend upon this resistance. If we place by the side of the 

 partial tube of wires, and in parallel circuit with them, a con- 

 ductor in all respects similar to that in the interior of the tube, 

 we have in the former brilliant sparks, but none whatever in 

 the latter. The former is unprotected, the latter is screened 

 by the tube of wires. We have in this an electrodynamic 

 analogue of the electrostatic experiment known as the electric 

 birdcage. I again altered the experiment, in the manner 

 depicted in fig. 1, p. 122. The two disks were placed so near 

 together that they formed, with the wires inserted between them, 

 a cage (A) just large enough for the reception of the spark- 

 micrometer. One of the disks, a, remained metallically con- 

 nected with the central wire ; the other, /3, was insulated from 

 the wire by means of a circular hole through its centre, 



