Eighteenth Annual Meeting, 



17 



The electro-magnet was a large one, designed with special reference to this work. 

 The diameter of each of the cores was 55 mm. and their length 380 mm. The pole 

 pieces of this magnet were so fashioned that the tube containing the iron and acid 

 could be brought into a tield of high intensity, (about 20,000 H.) 

 Fig. 1. Fig- 2. 



J>f 



I 



The magnet was actuated by means of a battery of grenet 

 cells, and a rheostat of simple construction made it possible 

 to reduce the strength of current at will. This instrument 

 consisted of a fine platinum wire, .03 mm. diameter and 65 cm. 

 long, which was stretched in a glass tube of 5 mm. diameter. 

 To the lower end of the glass tube was fastened one of soft 

 india-rubber, rather more than a meter in length, to the free 

 end of which a glass funnel was attached. The glass tube having been fastened ver- 

 tically to an iron standard, and the funnel being 

 supported in an adjustable ring attached to the lat- 

 ter, the funnel was filled with mercury, which thus 

 entered the glass tube containing the platinum wire. \ 

 By raising and lowering the funnel, this mercury 

 column could be made to occupy any desired por- 

 tion of the tube. That part of the platinum wire 

 not immersed in mercury, constituted the resist- 

 ance wire of the rheostat, and this resistance could 

 be given any desired value up to that offered by the 

 entire wire. To prevent the heating of the resist- 

 ance wire, the tube above the mercury was filled 

 with distilled water, the top of the tube being en- 

 larged to receive the water when expelled by the ris- 

 ing mercury. This rheostat is shown in figure 3. 



The strength of field was estimated by observ- 

 ing the swing of a delicately suspended galvanom- 

 eter needle induced by the withdrawal of a small 

 coil of known area from the field, and by compar- 

 ing this defiection with that produced upon the 

 same needle by a Weber's "earth inductor." 



With this apparatus the influence of magnetism 

 upon the passivity of iron was studied under vari- 



FlG. 3. 



ous circumstances and conditions. 

 2 



It was found very difficult, even with the knowl- 



