138 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XXIX.) 
cups of mercury. As both the needles are within the convolutions of this bar, an 
indicating bristle or fine wire of copper is fixed parallel to, and above them upon the 
same axis, and this, in travelling over the usual graduated circle, shows the place and 
the extent of vibration or swing of the needles below. The suspension is by cocoon 
silk, and in other respects the instrument is like a good ordinary galvanometer. 
3179 . It is highly important that the bar of copper about the needles should be 
perfectly clean. The vertical zero plane should, according to the construction, be 
midway between the two vertical coils of the bar, fig. 1 ; instead of 
which the needle at first pointed to the one side or the other, being 
evidently attracted by the upright portions of the bar. I at first feared 
that the copper was magnetic, but on cleaning the surface carefully 
with fine sand-paper, I was able to remove this effect, due no doubt to 
iron communicated by handling or the use of tools, and the needle then 
stood truly in a plane equidistant from the two coils, when that plane 
corresponded with the magnetic meridian. 
3180. The connexions for this galvanometer (3123, 3133.) were all 
of copper rod or wire 0‘2 of an inch in diameter*; but even with wires of this thick- 
ness the extent of the conductors should not be made more than is necessary ; for 
the increase from 6 to 8, 10 or 12 feet in length, makes a considerable difference at 
the galvanometer, when electric currents, low in intensity, are to be measured. It 
is most beautiful to observe in such cases the application of Ohm’s law of currents 
to the effects produced. When the connexions were extended to a distance, straight 
lengths of wire with dropping ends were provided, and these by dipping into cups 
of mercury completed the connexion and circuit. The cups consisted of cavities 
turned in flat pieces of wood. The ends of the connecting rods and of the gal- 
vanometer bar were first tinned, and then amalgamated ; after which their con- 
tact with the mercury was both ready and certain. Even where connexion had to 
be made by contact of the solid substances, I found it very convenient and certain 
to tin and amalgamate the ends of the conductors, wiping off the excess of mercury. 
The surfaces thus prepared are always ready for a good and perfect contact. 
3181. When the needle has taken up its position under the earth’s influence, and 
the copper coil is adjusted to it, the needle ought to stand at true zero, and appears 
so to do. When that is really the case, equal forces applied in succession on opposite 
sides of the needle (by two contrary currents through the coil for instance) ought to 
deflect the needle equally on both sides, and they do so. But sometimes, when the 
needle appears to stand at zero, it may not be truly in the magnetic meridian ; for a 
little torsion in the suspension thread, even though it be only 10° or 15° (for an in- 
different needle), and quite insensible to the eye looking at the magnetic needle, 
does deflect it, and then the force which opposes the swing of the needle, and which 
stops and returns the needle towards zero (being due both to the torsion and the 
earth’s force), is not equal on the two sides, and the consequence is, that the extent of 
