( &r. ) 



Physics. — „0« a 5-ceUar quadrant-electrometer and on the measu- 

 rement of the intensity of electric currents made ivith it.'' By 

 Prof. H. Haga. 



During the last years I was repeatedly obliged to measure with 

 accuracy the intensity of a constant current of about ten ampere. 



From the many methods which might be used, I chose that by 

 which the difference of potential is measured between the ends of a 

 known resistance, inserted in the circuit. As this method is excee- 

 dingly simple, capable of measuring currents of greatly different 

 intensities with an accuracy of Vio percent, it seems desirable to 

 me to draw the attention to this method. 



A good quadrant-electrometer is required. In 1893 Himstedt ^) 

 described a 4-cellar quadrant-electrometer; the needles were sus- 

 pended by a silvered fibre of quartz; the damping was obtained, 

 by hanging two vertical magnets at the lower end of the small rod 

 which bears the needles, the poles to opposite sides, so that they 

 could move within an annular space in a piece of copper. The 

 magnets not forming a perfect astatic system, a small directing force 

 was left , because of which Himstedt was obliged to make the 

 whole apparatus moveable round a vertical axis. 



It seemed to me that tliis dilBculty might be avoided, by rever- 

 sing the Himstedt method of damping, that is to say by hanging 

 a hollow copper cylinder movable in a magnetic field ^). A 5-cellar 

 quadrant- electrometer with this damping was constructed in the 

 physical laboi'atory at Groninghen. The principal part of this 

 apparatus is represented by fig. 1 ; the brass cylinder round the 

 quadrants and the cylindric case round the mirror are removed. 



The base of the instrument is a brass plate five mm. thick on 

 three levelling screws; the four quadrants are placed on the plate, 

 insulated by glass columns; one of them may be moved micronietri- 

 cally. The terminals are attached under the plate, in order to protect 

 them from dust as much as possible. They are perfectly insulated 

 from the plate by ebonite, glass and shellac. 



The whole apparatus for damping also is attached to the under- 

 side of the plate; it consists of a circular magnet three cm. high 

 (fig. 2), provided with the armaments a and ó; in the latter not- 

 ches have been filed, in which the poles of the magnet fit closely; 



') AVied. Ann. Bd. 50, p. 752, 1893. 

 -) Heildiitter, J!d. 19, p. S9G, 1895. 



