RECENT PROGRESS IN PHYSICS. 379 



In all electrostatic measurements it is quite certain that the most 

 important source of error is to he found in the gradual loss of the 

 electrical charge by the inductive action which the charged body has 

 on those near it, &c. The uncertainty which springs from this 

 source is certainly far greater than the errors of observation which 

 arise from adjusting and reading. From this point of view it seems 

 superfluous to apply to electrometers ol" all kinds a great array of 

 graduation, microscopes, &c. 



In the memoir already mentioned KoMrausch suggests the very ex- 

 cellent idea of employing the electroscopic power of the voltaic jiile as a 

 convenient measure for frictional electricity, or for comparing different 

 electrometers. 



A pile of a given number of elements, consisting of strips of zinc 

 and copper soldered together, immersed in small glass vessels contain- 

 ing distilled water, will have, if one pole be put into perfect connexion 

 with the ground, a constant tension upon the other pole, and, conse- 

 quently, will answer very well for comparing different electrometers. 

 In a long immersion (lasting over a week) the intensity will certainly 

 diminish, because the zinc becomes covered with oxyde, but the orig- 

 inal intensity may be restored by cleaning the metal with a file. 



KoMrausch obtained from the pole of such a pile the constant indi- 

 cation of 52° to 53° of his instrument during a whole week. After 

 the lapse of four weeks the indication had fallen to 46°, but it re- 

 turned to the original quantity as soon as the metal was cleaned by 

 filing. 



§ 15. Petrina's electroscope. — Petrina has constructed an electro- 

 scope in which he has substituted an electrophorous for the dry pile. 

 (New theory of the electrophorous, and a new resin-cake electroscope, 

 by Dr. Franz Petrina, from the Abhandlungen der Konigl. Bohmis- 

 chen GeseDschaft der Wissenschaften, Y. Folge, Bd. 4, Prag. 1846.) 

 The gold leaves are suspended between two metallic plates, one of 

 which is in connexion with the cover, the other with the insulated 

 dish of a small rosin-cake electrophorous. By a special contrivance 

 the mould, together with the cake, can be depressed, whereby one of 

 the plates (that connected with the cover) receives a positive and the 

 other a negative charge, so that the two plates here play the same 

 part as the pole plates of the dry pile in Bohnenberger's electrometer. 



It is, in fact, a very ingenious application of the electrophorous, and 

 if we did not possess the pile electrometer, we should welcome the 

 resin-cake electrophorous as an important addition to electrical appa- 

 ratus ; but whether this instrument, as compared with the pile elec- 

 trometer, will receive any practical consideration, I am very doubt- 

 ful. Petrina, indeed, thinks that it is easier to construct, because it is 

 easier to make a good cake of resin than a good Zaniboni \n\e ; but 

 the contrivance for raising and lowering the plate with the resin cake 

 may quite make up for this difference?. The only real advantage of 

 Petrina's apparatus is, perhaps, this : that it can never become use- 

 less from loss of power, because the resin, when it becomes weak, can 

 always be rendered electrical again. 



With reference to the rest of the contents of Petrina's memoir, it 

 should be discussed in another place ; but I will not return to it again 



