Mr Barlow on the Mathematical Laws ofEkctro-Magricthm. 281 
hold out to him some inducement to withdraw his attention from 
more alluring studies^ and devote it to the completion of an in- 
vention which will do honour to his country, and to the age in 
which we live. 
Edinburgh, August 1 . 1822. 
Art. XV. — Notice respecting Mr Barlow's Discovery qf the 
Mathematical Laws of Electro-Magnetism. * 
Mk Barlow of the Royal Military Academy, who has so 
successfully reduced the laws of induced magnetism to mathe- 
matical principles, has been equally fortunate in his experi- 
ments and investigations on electro^magnetism, having rendered 
this also a matter of computation. 
We are not exactly informed of the apparatus which he em- 
ployed in these researches ; but it appears that the machine. it- 
self was on the principle of Er Hare’s Calorimotor, and the 
other part consisted simply of a rectangle of stout brass wire, 
each side of which was four feet. One side of this rectangle 
was open, so as to make the connection with the battery:, and the 
other vertical side was passed through the centre of a table, di- 
vided into the several points of the compass, and round which, 
therefore, a magnetic needle might be placed at any azimuth. 
’The two horizontal sides of the rectangle might be slipped up 
and down on the vertical wires, whereby the length of the con- 
ducting part of the vertical wire might be changed at pleasure ; 
and the distance of the compass itself from the vertical wdre, 
might also, in like manner, be varied ad libitum^ by merely 
sliding to and from the centre. By these means, Mr Barlow 
was enabled first to determine the law of the electro- magnetic 
action, as it depended upon the distance, the length of the wire 
remaining constant ; and that law he found to be, that the ao 
■tion was inversely as the square of the distance, from every par^ 
* Our readers will be glad to learn that Mr Barlow is printing a second edition 
of his “ Essay on Magnetic Attractions,” which will embrace also the subject of 
Electron-magnetism , — En. 
