518 
Mr . Babbage on electrical 
not quite above the edge of the hot iron, and about io° from 
its centre. 
In order to get rid of the electrical effect of the glass or 
wooden supports, which I have called bridges, and which 
sometimes supported the needles, I placed a cork on the 
centre of the screen, immediately above the centre of the 
copper disc ; into this was fixed the point of a sewing needle, 
on which the agate caps of the needles rested. I conceived 
this arrangement necessary, because, when no screens were 
employed, and the distance between the needle and the re- 
volving plate was considerable, it sometimes happened that 
the needle followed the copper in its motion until it came 
over the supporting bridge, where it sometimes rested, instead 
of continuing its revolutions as long as the copper plate was 
kept in motion. 
Experiment 20. 
Using a wax card needle supported by a cork placed over 
the centre of the leno screen, and the lamp placed in the centre, 
under the copper plate, a slow motion was given to the lamp 
and plate. After some minutes, the needle had receded in a di- 
rection contrary to that of the motion of the plate through 35°. 
This experiment was repeated several times, and I observed 
that the needle, (if it moved at all,) generally went in an 
opposite direction to the plate ; and that its motion was often 
so slow as to be imperceptible to the eye, although it could be 
very distinctly perceived after the lapse of a short time, by 
its altered position. In various repetitions of this experiment, 
the needle slowly retrograded from 10 to go degrees.* 
* In one experiment which lasted above half an hour, and in which I had the 
advantage of Mr. Christie’s assistance, the needle slowly retrograded over 157 0 . 
In this case the motion was rather slower than that of the minute hand of a watch. 
