140 
the face or poles of the permanent magnet so as to slide off of one 
on to the other. In this case the effect will be double that of sep- 
arating it from a single pole; since if, in passing from the first pole 
it loses its magnetism, in passing on to the second it may be con- 
sidered as being demagnetized still further, since it is changed into 
the opposite magnetism. A similar result will be produced with the 
other leg of the horseshoe. To excite, therefore, the greatest 
possible amount of electrical induction, Mr. Saxton fastened the U 
to a revolving axis passing through its crown, to which a rapid ro- 
tation could be given by means of a driving wheel and pulley. In 
order, however, to obtain manifestations of the induced currents 
produced in the copper wire, the two ends of the coils were so 
soldered together as to give a single current in one direction through 
the entire length of the coils. One of the remaining ends was then 
permanently soldered toa circular disk fastened concentrically to the 
revolving axis by an insulating collar, with its plane perpendicular 
to it. This plate dipped into a cup of mercury. The other end 
of the wire was soldered directly to the revolving shaft or axis. In 
this arrangement the insulated disk formed one pole of the long 
wire and the revolving shaft the other; but as they were not con- 
nected no electrical excitement was observed when the bobbins 
were revolved. To make and break the connection at the proper 
moment, two wires were soldered diametrically opposite each other 
on a ferule which fitted tightly with friction on the revolving shaft. 
These wires standing out at right angles to the shaft were cut off at 
such a length that at each revolution the ends would plunge into 
the same cup of mercury with the revolving disk, and thus complete 
and break the circuit twice with each revolution of the bobbins. 
These wire points were then so adjusted by turning the ferule on 
the shaft as to cause them to enter and leave the mercury at the 
moment when the magnetism was increasing or diminishing most 
rapidly, and consequently when the current had the greatest in- 
tensity. 
*« With this instrument he was able to exhibit a brilliant electrical 
spark, to decompose water, to show the electrical light between 
charcoal points, and to give a rapid series of intense shocks. The 
instrument was exhibited to the public for the first time at the 
meeting of the British Association at Cambridge, in June, 1833, 
where it excited much interest. It was permanently placed in the 
Adelaide Gallery in August of the same year. The poet Coleridge, 
