72 
MR. HARRIS ON THE TRANSIENT MAGNETIC STATE 
( b ) In a tall receiver of about two feet high a reed similarly placed rotated 
at a distance of eighteen inches from the revolving disc, when the exhaustion 
was within six inches ; but it remained at rest at a foot distance, when the 
exhaustion was carried to within 0.4 of an inch. 
(c) The rotating disc being screened by a thin plate of glass or varnished 
paper fitted close upon a short hollow cylinder of glass or wood, the reed 
remained at rest, although placed within one fifth of an inch of the screen. 
( d) A cylindrical magnetic bar revolved rapidly in an exhausted medium, 
when within one fifth of an inch distance from the disc ; but only deviated 
from the meridian 40 or 50 degrees when the screen above mentioned was in- 
terposed. The disc in this experiment revolved at the rate of 400 revolutions 
in the minute. 
(e) Needles of glass, wood, and metallic substances not permanently mag- 
netic, remained at rest when the rotation exceeded 600 revolutions in a minute, 
whether suspended by filaments of silk, or otherwise placed on points and sup- 
ported on the glass screen. When the rotation exceeded 1000 revolutions in 
a minute, and the exhaustion was inconsiderable, they were sometimes slightly 
moved. 
(/) Rotating magnetic discs and bars induced motion in metallic discs 
freely suspended at one fifth of an inch distance, when their thickness was 
about four times as great as ordinary tinfoil, and when the rotation exceeded 500 
revolutions in a minute ; the rotating body being screened by any intervening 
substance except iron, which last, as already observed by Mr. Herschel and 
Mr. Babbage, completely intercepted the effect *. But light discs of wood and 
paper, and discs of paper covered with silver or gold leaf, remained at rest when 
within 2 3 oths of an inch distance from the rotating body. 
(g) A disc of iron or tempered steel did not communicate motion to any 
* Philosophical Transactions of the Royal Society for 1825. 
Although a sheet or two of tinned iron completely intercepts the influence of a revolving magnet on 
metallic discs generally, it does not appear to do so when the disc acted on is also of iron. I found, 
however, that a screen of sheet iron of about one fourth of an inch thick materially diminished the in- 
fluence even on an iron disc, so that the rotation of an iron disc by the influence of a revolving magnet 
is reduced nearly to that of a copper disc of the same dimensions when the iron screen is not present. 
This curious effect of screens may possibly lead to some further elucidation of the nature of magnetic 
influence. 
