Force of Magnetization. 293 



strength ; and it retained considerable residual magnetism 

 when the circuit was broken. By the following very simple 

 device, however, the residual magnetism was almost entirely 

 destroyed. A reversing switch of Poggendorff's pattern and 

 an ordinary telegraph key were placed in series in the magnet- 

 izing circuit. While the magnet was in function, a piece of 

 soft iron wire about l cm long was suspended by magnetic at- 

 traction from the underside of one of the pole pieces. The 

 wire served as an indicator of the magnetic condition of the 

 poles. Upon breaking circuit with the key the residual mag- 

 netism was sufficient to hold it in position. When, however, 

 the current was first reversed by means of the Poggendorff 

 switch, and then broken at the instant when the magnet was 

 passing through the condition of neutrality, the proper moment 

 being indicated by the dropping of the suspended wire, the 

 magnet was left thoroughly demagnetized. The interval of 

 time between reversal of the current and neutrality was about 

 two seconds. 



After each series of readings, the galvanometer was cali- 

 brated, the resistance of the iron cell was measured and the 

 strength of the magnetic field was estimated by a modification 

 of Rowland's method. The instrument used in most of these 

 measurements was Edelmann's form of the Wiedemann galvan- 

 ometer, read with telescope and scale. For some experiments 

 in which a high degree of sensitiveness was necessary a Thom- 

 son reflecting galvanometer of 2500 ohms resistance was used. 



It was found possible by this method, excepting when the 

 fluctuations in the initial electro-motive force of the cell were 

 very marked, to detect changes amounting to much less than 

 •00001 volts. 



We experimented with a variety of reagents, including ni- 

 tric, hydrochloric and sulphuric acids ; ferrous sulphate, ferrous 

 chloride, and ammonium chloride, in aqueous solutions, and 

 finally, sulphuric acid, to which potassium bichromate had been 

 added, and hydrochloric acid containing potassium chlorate. In 

 every case there was unmistakable evidence of the development 

 of a permanent electro-motive force due to the influence of the 

 magnet. The smallest effect, '000008 volts, was observed with 

 terminals in concentrated nitric acid, the iron being passive — 

 the largest effect in those solutions in which rapid oxidization 

 took place. In a solution consisting of dilute sulphuric acid 

 containing potassium bichromate, the electro-motive force of 

 magnetization amounted to '039 volts. In the same acid, the 

 concentrated sulphuric acid of commerce diluted with ten parts 

 of water, without the addition of the potassium bichromate, it 

 was only "0005 volts. Concentrated hydrochloric acid (sp. gr. 

 1*1768), gave "003 volts, the same acid, diluted with four parts 



