﻿Magnetizing and Astaticizing Galvanometer-Needles. 485 



Usually, owing to difficulties of construction, one member 

 will be slightly heavier and bence magnetically stronger, and 

 the magnetic pla les will be slightly inclined to each other ; 

 the amount and direction of which inclination can be ascer- 

 tained by noting the setting of the system with reference to 

 the magnetic meridian. 



The planes of the members should first be brought to 

 parallelism by a slight twisting of one of them on its support. 

 A good way to do this is to take two bar-magnets, N, S 

 (fig. 2), and bring first one and then the other up against 

 the side of the glass tube, as in fig. 2. 

 opposite the magnetically stronger Fig. 2. 



of the two members, which will be ,. . >^^^. r 



drawn against the wall of the glass ) W^ ^\l • 

 tube toward the magnet first brought """ | Jf~"" 



up, say X. If a right-handed twist is ||Jjj|L najfcjjjf 

 needed to bring the two planes to pa- SS3^^^0^Wllmlk 

 rallelism, the two magnets are held 



as shown, in front of the axis of the tube ; if a left-handed 

 one, behind the axis, as indicated by the dotted lines. Then 

 the magnet N is quickly removed and the member is drawn 

 toward S against the inclined wall of the tube, giving it a 

 blow which tends to twist it clockwise on the axis of suspen- 

 sion and at the same time to slightly weaken it. Repeating 

 this operation a few times will soon suffice to bring about the 

 required degree of parallelism unless the cement which 

 attaches the member to the staff be too hard and dry, in 

 which case a slight heating of the surrounding tube with a 

 hot block of copper or a Bunsen-flame will be necessary to 

 render the cement slightly plastic. 



Parallelism having been secured (which is determined by 

 the plane of the system lying in the magnetic meridian), the 

 relative magnetic strengths of the two members are tested by 

 determining the time of vibration. Generally, if the system 

 has been made up with a proper degree of care, the strengths 

 of the two members will be so nearly equal that it will only 

 require a very weak directing magnet placed some distance 

 above or below the system to secure almost perfect astaticism. 

 But if it is desirable to secure this without the use of such 

 a magnet, it will be necessary to add to the weaker of the 

 two systems a minute fragment of steel (a very short piece 

 broken off from one of the finest watch hair-springs answers 

 well), and then to remagnetize the whole as before, taking- 

 care of course to magnetize in the same direction. The whole 

 operation requires at the most but a few hours' work by an 

 experienced manipulator, and the success of this method may 

 be judged by the results which have been obtained by it< ase. 



