Irreciprocal Conduction. 127 



account of their labours before proceeding to describe our 

 own experiments. 



Section A . Historical and Critical. 

 1. Experiments of Erman. 



Among the many papers in the earlier volumes of Gilbert's 

 Annalen der Physik, which the invention of the voltaic pile 

 had directly inspired, will be found several by Prof. Erman 

 of Berlin. With the aid of the imperfect instruments known 

 at that time he made an investigation on the conducting 

 power of bodies ; and in his fourth research * there appears a 

 division of bodies into five classes, of which the following 

 schema shows the chief features : — 



All Bodies. 



Insulators. Class I. Conductors. 



I I 



Perfect Conductors. Class II. Imperfect Conductors. 



I 



I I 



Bipolar Conductors. Class III. Unipolar Conductors. 



Positive Unipolar Conductors. Negative Unipolar Conductors. 

 Class IV. Class V. 



To understand this classification it will be necessary to 

 describe the method of experiment employed by Erman in 

 order to find the class to which a body belonged. PN (fig. 1) 

 is a well insulated Volta's pile, P and N being the positive 

 and negative poles respectively. Connected with P is a gold- 

 leaf electroscope E^, and with N a similar electroscope E„. 

 The body (B) to be tested, which is permanently connected 

 with earth, is made to touch separately E^ and E n . If the 

 body belongs to Class I. neither of the electroscopes will be 

 affected, but the case is otherwise if B is a conductor. Let 

 us suppose the latter ; it will, therefore, be necessary to 

 ascertain to which of the other four classes the body 

 belongs. Permanent connexions are made (as represented 

 by the dotted lines of fig. 1) with B, and the behaviour of the 

 electroscopes is carefully noted. If there is a sudden and 

 complete collapse of the leaves of both E p and E n , the 



* " Ueber die fiinffaehe Verschiedenheit der Korper in Riicksicht auf 

 galvanisches Leitungsvermogen." Gilbert's Annalen der Physik, Band 

 xxii. 8. 14 (1K)G). 



