354 Electro-Magnetic Apparatus of Lieut. -Col. Offerhaus. 
with the diluted acid. The horizontal wire being in the mag- 
netic meridian, the apparatus is ready for experiments. As in 
the former instrument, the copper pole is positively, the zinc 
negatively electric. A magnetic needle of 0.168 metres long, be- 
ing placed at the distance of 43 millimetres, under the conductive 
wire, declined to the east, sometimes as much as 80°. Placed 
above the wire, and at the same distance, it went as much to 
the west, viz. 80°. Some philosophers indeed contend, that the 
deviation is always less when the needle is above than when un- 
der the wire. But so many circumstances, during the course of 
the experiments, will alter the deviation of the needle, that these 
changes cannot, with appearance of justice, be attributed to its 
position under or above the wire. I think at least that I have 
seen as strong deviations when the needle was above the wire as 
when it was under it. 
A remarkable feature in the effect of this spiral voltaic appa- 
ratus, is the strong adhesion of iron-filings to the conductive 
wire. If the zinc plate is new or well cleaned, the acid strong, 
and of course the galvanic process going on with great energy, 
then, if iron-filings, on a paper, are brought backward and for- 
ward, under, and close to the horizontal conductive wire of cop- 
per, the iron-filings will begin to stand erect, as if in the vicini- 
ty of a loadstone ; they will even adhere strongly to the copper 
wire when brought into contact with it, and fall down of course 
immediately when the wire is taken out from its pot. I must 
observe, however, that the diameter of the conductive horizontal 
wire appears to have a great influence on the phenomena. It 
should be neither too thick nor too thin. I found the experi- 
ments to succeed best when a wire was used of 5 millimetres 
diameter. 
I need scarcely remark, that this apparatus will very freely 
ignite platina wire. If it is Jth of a millimetre in diameter, it is ig- 
nited immediately, by stretching it from one of the copper plates 
to the zinc. 
The intensity of the magnetic force will, as might have been an- 
ticipated, be materially increased by placing the needle under or 
above the conductive wire. A magnetic needle in its ordinary 
state, without the influence of the galvanic apparatus, was found 
to make 12 oscillations in one minute ; being placed at the dis- 
