310 



Mr. W. Hibbert on a 



At the upper end of the guide-rod g is an arrangement tor 

 allowing the ring to fall whenever the experimenter desires. 

 The upper end of g is bored out (fig. 3), and a horizontal slot 

 is cut in the thin wall thus made. In the inner 

 space and connected with the milled head is a rod, 

 from the side of which projects a tooth t passing 

 through the slot, whilst from the lower end 

 passes a spiral spring also fixed into the body 

 of g. The tooth t is thus kept in a definite 

 position, from w T hich it can be moved aside 

 either by a rotation of the milled head, or by 

 the upward movement of the boss connected 

 with the ring. On the inner edge of the boss 

 is a slot through which t can pass, but cut 

 slightly on one side of the zero position of the 

 tooth. When an electromagnetic impulse is 

 desired the coil is raised so that the boss, and 

 therefore the ring, rests on t. The position of 



t and the height of the 



are so chosen 



that when the ring is at rest on t the coil is 

 altogether above the gap. By a simple rotation of the milled 

 head the coil can be made to fall through the field at any 

 desired moment. The electromagnetic impulse then given to 

 the circuit is, of course, equal to the number of lines inter- 

 linked with the coil during the fall, multiplied into the number 

 of turns of wire in the coil. 



Three instruments of the type just described have been 

 made and tested. The means adopted for testino- are not the 

 best that could be specified for the purpose. They were 

 chosen because they give reasonable accuracy, and yet allow T 

 of a fair number of observations being taken in a limited time. 



The method was to compare two throws of a galvanometer- 

 needle, one produced by the discharge of a condenser, the 

 other produced by the magneto-inductor. 



The condenser employed was a mica condenser of 0*333 

 microfarad capacity. The potential difference for charo-ino- 

 was obtained from four accumulators, whose electromotive force 

 was determined by comparison with a Latimer-Clark cell, the 

 comparison being made by the potentiometer method. 



Having first taken a fair number of observations from the 

 condenser, a corresponding number were taken from the 

 magneto-inductors, the resistance in circuit with each being- 

 adjusted till the throw was practically the same as that 

 obtained from the condenser. 



I decided on this because it allowed me to use an ordinary 

 damped reflecting-galvanometer. The object being simply 



