an Electric Micrometer. 539 



observer cnn have it attached to his head and hear the contact 



made, while at the same time he is watching the disk (Gr) for 



its position. In this manner the position of the "break" of 



contact (a) and (6) can be observed. 



Suppose now a steady current passes in the receiver (Rec), 



the diaphragm will move to a new position and remain there; 



then the observer can quite easily follow, and measure, its 



movement. 



The wooden frame carrying the apparatus was heavily 



weighted with lead and supported on soft rubber balls, to 



insulate it against vibrations. 



The ratio of the lever-arms was about 10 : 1. By using a 



telescope, each graduation on the disk (Gr) could be divided by 



eve into ten parts ; each of these corresponded to a movement 



of the screw-end of 5013-0 °^ 2 mm., or 10 ~ 5 cm., or at the 



other end of the lever to 10 ~ 6 cm. about. 



Second Form*. — In the next attempt (fig. 2, p. 543), the 



single brass lever was replaced by three levers of aluminium, 

 workinof in co-operation, so that the joint leverage ratio could 

 be made 1000 : 1. 



The levers are made of aluminium bar ( \ in. x ^ in. section) 

 about 15 inches long ; the pivots are of steel, their ends being- 

 pointed to rather acute cones. The lower bearings for the 

 lever-pivots are provided in brass plates let into brackets, 

 whilst the upper bearings are turned in the screw-ends. The 

 brackets which carry these top bearings are made of stout 

 brass bar, bent twice at ri?ht angles and firmly screwed to the 

 frame and to the wooden brackets. 



The screws mentioned be 'ore have lock-nuts bearing on 

 the brass brackets so that the pivots can be tightened up in 

 their bearings as required. Each lever is balanced by a lead 

 weight screwed to its short arm, the object being to have the 

 centre of gravity of each lever over the lower bearing so as to 

 have no side pressure and consequent wearing of the upper and 

 lower bearings. The lever-ends are kept firmly in contact 

 with their bearings by the action of the helical brass springs 

 attached to the levers and to the frame ; these springs are 

 fairly strong, so as to render the pressure throughout the 

 lever-system firm and reliable in all positions. 



This point is of obvious importance, for any backlash in the 

 screw or looseness of the pivots in their bearings, would be 

 nullified by the action of the springs ; so that we should still 

 have firmness in the lever-system. 



The bearings by which the ends of the second lever press 

 on the other two are moveable, so that we can obtain different 

 * Exhibited at the Royal Society Conversazione on May 8, 1900. 



2Q2 



