122 Rev. W. Ritchie on the Power of an Electro- Magnet 



torrents of sulphurous acid gas are then evolved by boiling : the 

 nitric acid is here too weak to decompose the sulphurous acid 

 gas, absorbed by the water of the house. If W denote the 

 colour to be white, D dark, X traces of nitric acid, and O its 

 absence, then the following will denote the number of drawings 

 or lead kettles-full from lead-houses under my superintendence. 

 I tested each lot to ascertain the presence of nitric acid. 

 11 times W and X 



1 — W — o 



10 — D — O 



2 — D — X 



One house gave 7 D and O 



4 W and X ; its product of acid was 2*78 

 pounds for one pound of sulphur burned. 

 Another house gave W and X all the time; its product was 

 2*67 of acid for one pound of sulphur. 



XXV. On the Power of an Electro-Magnet to retain its Mag- 

 netism after the Battery has been remoxied. By the Rev. 

 William Ritchie, LL.D. F.R.S. Professor of Natural 

 Philosophy in the Royal Institution of Great Britain and 

 in the University of London. 



To Sir David Brewster. 

 Dear Sir, 

 A LMOST as soon as the mode of making an electro-mag- 

 -*•*- net was invented, it was observed that the lifter of soft 

 iron, did not fall off when the contact was broken, but re- 

 mained suspended for a considerable, time after. Mr. Watkins 

 has an electro -magnet which retains its lifter for days, and 

 will even afford a magneto-electric spark for a long period 

 after the contact with the battery has been broken. But what 

 is a fact with one electro-magnet is not so to the same extent 

 with another, and I have succeeded in getting a very powerful 

 electro-magnet which possesses this property in a very inferior 

 degree. One obvious circumstance which must modify the re- 

 taining power, is the quality of the iron and its degree of soft- 

 ness. The harder the iron, the more powerful will be the re- 

 tentive force, and the longer it will continue. But the most 

 remarkable circumstance which modifies the retaining power, 

 and one which as far as I know has not been previously ob- 

 served, is the length of the magnetic circuit. When the electro- 

 magnet is very short, and the poles near each other, the re- 

 taining power is exceedingly small. When the magnet is very 

 long, the retaining power is very great. I have three magnets 



