580 Prof. J. E. Ives and Mr. S. J. Mauchly on 



with closely fitting, split, bar J rubber tubes, having shoulders 

 on them to keep the two parts of the brass tube the proper 

 distance apart. At b, two binding posts were soldered to the 

 ends of the brass tubes. The tubes forming the square were 

 carefully soldered together at the corners, q, c 2 , c 3 , c 4 . The 

 insulated copper wire within the tubing had a diameter of 

 0'1007 cm. The ends of this wire were brought out at a. 



The sliding conductor, s 1 s 2 , was attached securely to the 

 lower side ol a wooden bar, and consisted of a stout brass 

 wire, *318 cm. in diameter, soldered at its ends to square 

 brass plates, 3' 25 cm. on a side, to which were attached 

 half-cylinders of brass , 3*20 cm. long, engaging with the 

 brass tubing. It could be moved parallel to itself through 

 any desired distance, between wooden stops attached to the 

 wooden bar a b. 



At a was a closely-fitting split brass sleeve, about 3 cms. 

 long, sliding upon the brass tube, which could be slid over 

 the gap at a, forming a conducting bridge between the two 

 tubes t\ a and c 2 a, when desired. 



The binding posts at b were connected to a two-coil Du 

 Bois-Rubens armoured galvanometer made by Siemens and 

 Halske, having a resistance, with the two coils in parallel, of 

 about 2*5 ohms, a period of about 4 seconds, and a sensi- 

 tiveness, as adjusted for this experiment, of about 2xl0 -8 

 ampere for one millimetre at a distance of one metre. This 

 galvanometer is described in the Zeitschrift fur Instru- 

 mentenkunde , Jahrg. 1900, p. Go. 



The arrangement of the circuits is shown in fio-. 2, 

 where 



*i s 2 is the slider ; 

 G, the galvanometer ; 

 Hi, a resistance in the tube circuit to give a convenient 



deflexion ; 

 G i c 2 c z Q> the square of brass tubing ; 

 B, battery in the wire circuit ; 

 K, l^y 



A 1? ammeter ,, „ and 



B> 2 > a rheostat „ „ to adjust the current to a suitable 



value. 



The experiment consisted in varying the flux, due \o the 

 earth's field, through the rectangle s Y s 2 c 3 c 4 by moving the 

 slider s x s 2 , and then calibrating the galvanometer by making 

 or breaking a current in the wire circuit, and noting the 

 deflexion produced in the tube circuit. 



• The procedure was as follows :— With the tube circuit 

 open at a, and the wire circuit open at K, the slider was 

 moved through a suitable distance between stops, and the 



