PERIODS OF ELECTRICAL VIBRATIONS. 207 
cms. in diameter and one of copper wire 0°33 cms. thick. 
An open circle was compared with the rectangles in turn, 
and was in tune with each of them when its circumference 
was 380 cms., the perimeter of each rectangle being 886 
cms. and the width 30 cms. 
Permeability of the mediwm surrounding the circuits. 
—With the rapid alternations of current used in these 
experiments it is not to be expected that the permeability 
oi the material of the circuit would have a considerable 
effect on the period of vibration (see St. John, loc. cit.). 
The permeability of the medium outside the wire on the 
other hand is of primary importance in this connection. 
This may be readily shown by surrounding the middle por- 
tion of a wire, where the current is concentrated, with fine, 
well insulated iron filings, and comparing the period of 
vibration with that of the wire in air. In the case tried, 
the central part of a straight copper wire 0°33 cms. in 
diameter, covered with paraffined paper, passed centrally 
through a glass tube 70 cms. long and 1 cm. internal 
diameter, the tube being filled with paraffined iron filings. 
The wire under these circumstances, when 345 cms. long, 
had the same period of electrical vibration as a copper wire 
of the same gauge wholly in air 370 cms. long. 
RESULTS. 
In the following tables under the headings “‘ perimeter 
of rectangle,” and ‘“‘length of straight (or curved) wire,”’ 
the respective lengths of the two circuits, when the elec- 
trical oscillations connected with them are in unison, are 
placed in the same row, the fundamental mode of vibration- 
only being investigated. The rectangles have been made 
with sides of thin brass wire 0°04 cms. in diameter, the | 
ends being of copper wire 0°33 cms. thick. The other 
