﻿444 Prof. John Trowbridge on the Change of Period 



Repeated measures between the same number of oscillations 

 give closer results ; but one is apt to set the measuring 

 instrument each time on the same points on the negative. It 

 is evident that measuring each time the space between a 

 different number of oscillations gives the fairest result. It is 

 surprising how close one can set the measuring instrument 

 upon the serrations of the negative. 



It will be noticed that the ratio between any two determina- 

 tions of the time on the time circuit is the same as that 

 between the corresponding times on the trial circuits. When 

 iron wire of suitable diameter, however, was substituted for 

 copper wire of the same diameter and same geometrical form 

 in the trial circuits, the ratio of the determinations of time in 

 the time circuit and the ratio of the oscillations in the iron 

 circuit were no longer constant. This inconstancy 1 desire 

 to dwell upon as my strongest proof that the period of elec- 

 trical oscillations on iron wires is not the same as that of 

 oscillations on copper wires of the same geometrical form. 



The arrangement of a suitable iron circuit gave me con- 

 siderable trouble. The problem was to obtain a sufficient 

 length of iron wire to show any effect of change in periodicity, 

 and also to obtain a sufficient amount of self-induction, in 

 order that the distances between the oscillations on the photo- 

 graph should be measurable. The strong damping effect of 

 iron did not permit of my using more than four or five metres 

 of wire. It was not a simple matter to arrange two circuits, 

 one of iron and one of copper, which would have exactly the 

 same geometrical form. After many trials I arranged my 

 trial circuit as follows : — 



A cylinder of very porous wood, 11*5 centim. in diameter, 

 15 centim. long, was boiled in paraffin, and a spiral was cut 

 upon its surface. The turns of the spiral were 1 centim. apart. 

 On this cylinder and in these spirals the wires under examina- 

 tion were wound. After a determination had been made with 

 a copper wire it was unwound from the cylinder and an iron 

 or steel wire was wound in the grooves occupied by the copper 

 wire. Exact geometrical similarity was thus obtained with 

 good insulation. Several hundred determinations were made 

 with wires of different sizes. With iron wires larger than 

 •0312 inch in diameter, no marked change in period could be 

 perceived. If a great number, however, of photographs were 

 measured, an inconstancy of ratios was noticed which never 

 appeared when the copper circuits were compared. It seemed 

 as if at certain times the iron exerted a magnetic effect and 

 at other times failed to do this. The most marked changes 

 in period I obtained with iron wires of "0312 inch in diameter. 



