Electro- Magnetic Apparatus of Lieut. -Col. Offerhaus. 353 
which are quite analogous with those made, and at present known 
to every philosopher. We wanted, however, if possible, to obtain 
stronger effects, by exposing, in a similar apparatus, larger sur- 
faces of zinc and copper to mutual action. My friend, Lieut-Col. 
Offerhaus of the Engineers, contrived the apparatus, of which I 
subjoin a drawing. Perhaps you may find this not unworthy of 
a place in the Edinburgh Philosophical Journal, as it may possi- 
bly be brought, in the hands of English artists, to greater perfection. 
Plate X. Fig. 1. shews its external appearance; Fig 2. the Plan; 
and Fig. 3. the Section through the axis of the apparatus. The 
whole, as Fig. 1. shews, is contained in a cask of 51 centimetres 
high, and in diameter 38 centimetres at top. In the centre of 
Fig. 2. is represented a wooden cylinder, which constitutes the 
nucleus of the whole. To this cylinder is nailed a copper plate 
4.67 metres long and 0.40 metres high, of course its superficies is 
of about 1.9 metre, say about 2 square metres. This copper-plate 
is wound, spiral-like, round the nucleus, as shewn by the white 
colour in Fig. 2. In the intervals of the helices of this spiral, a 
zinc plate is placed, long 3.37 metres and 0.4 metres high. Fig. 2. 
shows also how this zinc follows the spiral windings of the cop- 
per one ; but they are prevented from touching each other imme- 
diately by wooden sticks or rods, of 1 centimetre in diameter, 
stuck between both metals, as the small circles in the same figure 
are intended to show. The zinc is marked by a darker shade, to 
distinguish it from the copper. How these sticks separate the 
copper from the zinc may also be seen in Fig. 3. 
From that part of the zinc plate which is nearest to the wooden 
cylinder or nucleus, projects a copper wire, and from that part 
of the copper plate nearest to the side of the cask which contains 
the whole, projects a second copper wire, as seen in Figs. 1. and 2. 
On these the conductive wire AB, Fig. 2. is laid, and may 
be placed in the magnetic meridian. Both wires, which spring 
from the copper and zinc plates, have at their ends in A and B 
small pots, to contain a little mercury ; and the ends of the con- 
ductive horizontal wire have small pins, to enter into these pots, 
and thus being immersed in mercury, make the contact more 
close. The horizontal conductive wire may be removed as oc- 
casion requires. This spiral apparatus being thus arranged in 
its tub or cask, the interstices between the metals are filled 
