1 70 Prof. Tyndall on the Absorption and 



In the sequel I shall refer to circumstances which induce me 

 to conclude that the result obtained by Dr. Franz is due to an 

 inadvertence in his mode of observation. These are the only 

 experiments of this nature with which I am acquainted, and 

 they leave the field of inquiry now before us perfectly unbroken 

 ground. 



§ 2. At an early stage of the investigation, I experienced the 

 need of a first-class galvanometer. My instrument was con- 

 structed by that excellent workman, Sauerwald of Berlin. The 

 needles are suspended independently of the shade ; the latter is 

 constructed so as to enclose the smallest possible amount of air, 

 the disturbance of aerial currents being thereby practically 

 avoided. The plane glass plate, which forms the cover of the 

 instrument, is close to the needle ; so that the position of the 

 latter can be read off with ease and accuracy either by the naked 

 eye or by a magnifying lens. 



The wire of the coil belonging to this instrument was drawn 

 from copper obtained from a galvano-plastic manufactory in the 

 Prussian Capital ; but it was not free from the magnetic metals. 



In consequence of its impurity in this respect, when the 

 needles were perfectly astatic they deviated as much as 30° right 

 and left of the neutral line. To neutralize this, a " compensator " 

 was made use of, by which the needle was gently drawn to zero 

 in opposition to the magnetism of the coil. 



But the instrument suffered much in point of delicacy from 

 this arrangement, and accurate quantitative determinations with 

 it were unattainable. I therefore sought to replace the Berlin 

 coil by a less magnetic one. Mr. Becker first supplied me with 

 a coil which reduced the lateral deflection from 30° to 3°. 



But even this small residue was a source of great annoyance 

 to me j and for a time I almost despaired of obtaining pure cop- 

 per wire. I knew that Professor Magnus had succeeded in ob- 

 taining it for his galvanometer, but the labour of doing so was 

 immense*. Previous to undertaking a similar task, the thought 

 occurred to me, that for my purpose a magnet furnished an 

 immediate and perfect test as to the quality of the wire. Pure 

 copper is diamagnetic ; hence its repulsion or attraction by the 

 magnet would at once declare its fitness or unfitness for the pur- 

 pose which I had in view. 



Fragments of the wire first furnished to me by M. Sauerwald 

 were strongly attracted by the magnet. The wire furnished by 

 Mr. Becker, when covered with its green silk, was also at- 

 tracted, though in a much feebler degree. 



I then removed the green silk covering from the latter and 

 tested the naked wire. 7/ was repelled. The whole annoyance 



* Pogg. Ann. vol. lxxxiii. p. 489 ; and Phil.' Mag. 1852, vol. iii. p. 82. 



