Mr Barlow’s Electromagnetic Experiments. S69 
This part of the apparatus is shewn in Plate VII., Fig. 1., 
where “ AB is an upright stand, placed near the poles of the 
battery ; ab, cd, are two staples of stout copper-wire, driven 
into the upright, the two ends at b and c passing quite through, 
as shewn at C and Z ; and on which two wires are fastened by 
spiral turns, and with which the communication is made with 
the poles of the battery ; ef, gh ,' are two copper- wires of the 
same dimension as the staples, each four feet long, having their 
ends flattened and drilled so as just to enable them to slide 
freely upon the wires ab, c d, and the vertical wir efh, also 4* 
feet in length, which passes through a hole in the top of the 
table FGHI, and so tight as to render it perfectly fixed. On 
the plane of the table, which is two feet in square, the circle 
NESW is described about the centre o, and divided into the 
points of the compass and smaller divisions; NS, is an index 
or box-ruler, through which the wirejf^ passes, so that the for- 
mer may be turned freely about the latter, and set to any pro- 
posed azimuth. On this ruler is placed the small compass d, 
by means of which the deviation at any time may be taken ; 
d' is another compass placed on the top of the support L d' 9 
and is intended to remain fixed in its place, in order to serve as 
a standard for estimating and comparing the power of the bat- 
tery at different times. 
“ For the principal experiments this apparatus is placed so that 
the plane of the rectangle of wires is perpendicular to the mag- 
netic meridian ; because in this position the horizontal wires 
being east and west, they have no effect in deflecting the needle 
from its direction (at least there is only one exception to this, 
which will be noticed hereafter), and consequently all the effect 
produced upon the needle during the rotation of the index in 
the circle NESW, is due to the vertical wire only, except so far 
as the horizontal wires may increase or diminish the directive 
power of the needle. This, however, in the cases to which we 
shall refer, is very inconsiderable. 
(6 But in order that we may know precisely what part of the 
change of deviation between one situation of the compass and 
another is actually due to that change of position, recourse must 
be had to the standard compass, which, always remaining fixed 
VOL. VIII. NO. 16. APRIL 1823. a a 
