Instrument for Measuring Magnetic Fields. 187 



In principle it is the inversion of the D'Arsonval galvano- 

 meter ; the torsion necessary to restore a coil, through which 

 a constant current circulates, to its normal position, parallel 

 to the direction of the lines of force, furnishing readings pro- 

 portional to the field at the position of the coil*. 



A diagram of the instrument is shown in fig. 1 (Plate III.). 

 A B is a small coil, oblong in shape, wound of No. 44 B.W.Gr. 

 copper wire, and suppported half on each side of a sheet of 

 mica. It is suspended from each end by strips, 10 centim. 

 long, of rolled German-silver wire, each strip having a loop 

 which is passed over a small brass hook riveted on to the 

 mica, and in electrical communication with a terminal of the 

 coil. The strip C A is in electrical connexion with the case 

 of the instrument at C, whilst the strip D B is insulated from 

 it at D by an ebonite plug, attached to the torsion-head E. 

 Inside this latter is a commutator for automatically reversing 

 the current, so as to take readings on each side of the zero. 

 It consists of four semicircular strips of copper, cross connected 

 as shown in fig. 2, a and b being connected to the two battery 

 terminals. Two springs, one soldered to the case, the other 

 insulated from it, but connected to the end of the suspending 

 strip D B, press on these semicircular strips. When the 

 torsion-head is at zero no current passes, the springs then 

 being at c and d respectively (fig. 2). To take a reading the 

 torsion-head is turned, thus sending the current through the 

 coil. Should the latter be deflected in the wrong direction, 

 the current can be reversed by means of the plug contact, P, 

 attached to the battery leads. Readings are taken on each 

 side of zero in order to eliminate any error due to imperfect 

 balancing of the coil ; an aluminium pointer G, attached to 

 the coil, being always brought by the torsion to the zero 

 position on a small scale. 



In order to obtain at once a spring-suspension and an 

 adjustment for the torsion of the strip, a particular form of 

 geometrical slide is used. A A' (fig. 3) is a thick brass tube, 

 turned at B B / to a slightly conical plug to fit the tube of 



* After the completion of this instrument our attention was called to 

 some experiments of Messrs. Siemens and Halske, in which the same 

 principle was used. 



" In order to measure the intensities of the rotary field, a coil was 

 hung in the centre of the ring, in* such a way that its magnetic axis was 

 perpendicular to the measured direction of the resulting magnetic axis of 

 the ring. The coil was then excited by a continuous current, and was 

 kept in position by a spring. The torque uf the spring served as a 

 measure of the intensity." "Deduction and Experiments on Rotary 

 Currents." A. du Bois Reymond, 'Electrical Review/ June 5, 181)1, 

 vol. xxviii. No. 70(3. 



