2 
PEOFESSOE TYNDALL ON THE ABSOEPTION AND 
result obtained by Dr. Feanz 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 galvano- 
meter. My instrument was constructed by that excellent workman, Saueewald of 
Berlin. The needles are suspended independently of the shade ; the latter is con- 
structed 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. Beckee 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, and for a time I 
almost despaired of obtaining pure copper wire. I knew that Professor Magnus had 
succeeded in obtaining 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 purpose which I had in view. 
Fragments of the wire first furnished to me by M. Saueewald were strongly attracted 
by the magnet. The wire furnished by Mr. Beckee, when covered with its green silk, 
was also attracted, though in a much feebler degree. 
I then removed the green silk covering from the latter and tested the naked wire. 
It was repelled. The whole annoyance Avas thus fastened on the green silk ; some iron 
compound had been used in the dyeing of it ; and to this the deviation of my needle 
from zero was manifestly due. 
I had the green coating removed and the wire overspun with white silk, clean hands 
being used in the process. A perfect galvanometer is the result. The needle, when 
released from the action of a current, returns accurately to zero, and is perfectly free 
from all magnetic action on the part of the coil. In fact while we have been devising 
* Poggexdoeff’s Annalen, vol. Ixxxiii. p. 489 ; and Phil. Mag. 1852, vol. iii. p. 82. 
