148 PROCEEDINGS OF THE AMERICAN ACADEMY 



galvanometer coils, through which by means of a double key the 

 current from a small thermopile could be sent in either direction, 

 served to damp the oscillations of the needle after a throw had been 

 given. 



A few of our measurements were made with the help of another 

 four-coil galvanometer (B), of 3000 ohms' resistance. The period of a 

 complete swing of its needle was 52.7 seconds, and the ratio of succes- 

 sive swings 1.068. With scale placed at a distance of one meter from 

 the mirror, one microcoulomb when discharged through this galva- 

 nometer gave a throw of 45 mm. 



For intervals between 0.01 sec. and 0.5 sec, the apparatus shown 

 in Figure 1 was used. It consists of a pendulum with an iron bob, A, 

 eight inches long, two inches in diameter, and weighing 7 lb., and 

 a brass rod ^ inch in diameter weighing 2.8 lb. The axis of suspen- 

 sion is formed by a brass rod 2^ inches long held between pointed 

 screws. 



Upon this axis and concentric with it are placed two ebonite disks, 

 F, G inches in diameter and | inch thick. The face of each of these 

 disks is armored through about half of its circumference with a strip 

 of German silver ■£% inch in thickness, held by rivets passing through 

 the disk, and the edge of the armature is turned true with the edge of 

 the disk. 



The disks are placed with their armored faces adjacent, but separated 

 by an ebonite washer 5 inches in diameter and ^V inch in thickness. 

 One of the disks is fixed to the pendulum rod, while the other can be 

 revolved about their common axis so that the two German silver strips 

 may overlap to any desired extent ; the arc a, attached to the movable 

 disk, serves to clamp the two disks firmly together, and also to deter- 

 mine approximately the amount of the overlapping. To each of the 

 German silver strips is joined a wire, b, which dips in a mercury cup, g, 

 connected with one of the two points between which the circuit is to 

 be made ; to avoid as far as possible any interference with the motion 

 of the pendulum, these mercury cups are placed very close to the axis 

 of suspension. 



The pointed screws upon which the pendulum swings are held in a 

 frame, c c, of wrought iron, which is itself swung between two pointed 

 screws, d d, perpendicular to the axis of suspension of the pendulum. 

 By rotation of the frame c c upon these screws, the axis of the pen- 

 dulum is easily levelled. 



To the frame c c is attached a pillar, e, carrying a copper wire, / 

 5 % of an inch in diameter, in such a position that its end runs with 



