482 



iiEruiiT — 1884. 



^'!i: 



i !1 



fiulteil in tho conclnsion that a coiulcmser mado of two difforont metal- 

 KhowL'd nearly tho same Volta elTect, whether theatmoHjdicrosurroundii);,'' 

 tho ])latc8 was air or hydrojTon, 



If it 1)0 assumed that tho experiments of IJrown and Schultzc-Rerirr 

 establish their point, and that I'cUat's apparatus for different {jjases (tig. !>) 

 is satisfactory (rather a largo assumption), I am unablo to reooncilo tin 

 discrepaney, exeept by suggesting that I'ellat did not take siillioient pains 

 to remove tho condensed air sheet originally on both his plates. It is ot 

 course just possible that tho ih'lfercnci' between tho potentials of tho twn 

 metals might bo the same in two gases tiioiigh tho absolute potential c i 

 both was dill'erent, but it is improbable. 



In this connection I must notice also a rather long memoir ' bv l)r. W. 

 von Zahn, published in l!^H2, which reviews the whole subject, and de- 

 scribes an elaborate series of measui'ements made with an apparatus 

 something like what ono nn'ght suppose Ayrton and Perry's to become il 

 it were arranged for use in (lilferent gases and in vacuo. He refers witli 

 admiration to Pellat's work in the preface, and I do notsuppo.so imagine- 

 that his own numerical determinations can compiii-o with Pellat's I'oi' 

 accuracy where they overlap, seeing that he only makes use of a sort ol 

 combination of Kohlranach's and Hankel's methods, with a llankcl elec- 

 trometer as a measuring instrument. ^ Ho has tried, however, a larLri' 

 number of substances as well .as ordinary metals such as ])owdere(! 

 antimony, iron and nickel I'educed. by hydrogen, many kinds of carbon, 

 FoaOj, manganite, pyrolusito, copper oxide, lead ' byperoxide,' iron 

 glance, and other minerals. H<> lias measured tho Volta effect in various 

 gases and at difl'ereiit pressures, and finds, like Pellat, that it does mti 

 appreciably vary. 



He has also examined the effect of tenipcratnrc! on the Volta efl'eci. 

 though he appeal's to think that this ought to bear some close relation to the 

 phenomenon of Seebeck, a natural mistake many years ago when made bv 

 Avenarius, whom it led most happily though fortuitously to tho true, am! 

 by him experimentally verified, law of K.^I.F. in a thermoelectric circuit.' 

 However, Zahn finds that experiment lends no support to this view, ami 

 says that a larger series of results must bo obtained before basing a theory 

 on them. Voti Zahn is a confirmed contact theorist, and he victorioush 

 assails several experiments supposed to be distinctly in favour of a chemi- 

 cal view of the Volta effect. He says he publishes his results because of the 

 extraordinai'y discoveries being propounded by Professor Exner (such a^* 

 that a thermopile will not work in a vacuum),* and because of the vagnc 



* Untersuchitnaen iibcr ContuctrlehififHiif, von Dr. W. v. /.aim, Lcijixii,'. Tiubiin, 

 1882. 



- A llankcl electromotor is a modiiic ation of IVilincnbori^fr's, in wliitli a liatt'n 

 with middle to earth is subslinitod for the dry jiile ; the plates on citlier side of tlir 

 \io\i\ leaf arc niinutely adjustable, and the nuitions of tlic i;()ld leal' are rcinlliy;i 

 juicroscopc. It is soiuetimes prcfcrvcd to a (piadrant for its small capacity and dcii'l 

 quick motion ; it can be made very sensitive, but it can liardly be a satist'acto'.y 

 measuring instrument. I'ellat used it, but only as an electroscope. 



* Avenarius: ' Die Thcriuoelektricitilt, ihreni Ursprun.ye nach, ;ds idi'utisch im' 

 der Contactelektricitiit bctrachtet,' Pofi;/. Ann. cxix. 180:!. iScc also J'o;/!/. Ann- 

 cxxii., wlicro lie proceeds to calculate Volta ctTocts from tluM-moclectrii; data. 



■• I have boon unablo to find tiiis <'xtraordinary statement in j'ixncr's works, b"f 

 it is quoted again by Ayrton and I'erry, P/iil. Jfnf/., 1881, p. 4!>. Exner scenis tc 

 have said that tlie tlicrmocleetric jiower of bismulh-antimony is destroyed '>> 

 imtnersinfr tlie i)ile in piu'o nitrop:en, and Younc; <if I'rincetown takes tlie trouble i" 

 exammc wliether it is so experimentally (sec I'/iil. Muf/. x. 1880, p. 150), and linil> 



