496 Wadsworth — Interrupter for Large Induction Coils. 



and Miss E. F. Fisher, students in the Geological Department 

 of the Massachusetts Institute of Technology. We also take 

 pleasure in acknowledging our great obligation to Mr. W. W. 

 Herman for freely placing his extensive collection at our dis- 

 posal. To his zeal as a collector we owe not only a goodly 

 number of the new species recorded in this paper ; but many 

 other species in his collection are new for the localities in 

 which he found them. 



Aet. LXYI. — An Improved Form of Interrupter for Large 

 Induction Coils ; by F. L. O. Wadsworth. 



Every one who has had occasion to use a large induction 

 coil knows the difficulty of keeping the contact breaker in 

 good working order. Of the two forms of interrupter in 

 common use, i. e., the spring contact form and the well known 

 Foucault form, the latter is best suited for large coils, because 

 the "break" may be made to take place under water or 

 alcohol and the rapid oxidation of the contact surfaces is 

 thereby much reduced. Even under the best conditions, how- 

 ever, it is difficult to secure uniformity of working for any 

 length of time, and this, together with the necessity for fre- 

 quent cleaning of the parts and renewal of the alcohol and 

 mercury, renders the apparatus anything but satisfactory, 

 when continued use with heavy currents is desired. 



In the last number of this Journal Mr. St. John* describes 

 a form of mercury contact interrupter similar to the ordinary 

 Foucault form, except that the plungers are positively driven 

 by means of cranks geared to a small electric motor, instead 

 of by the more uncertain action, of an electro magnet. This 

 undoubtedly will give more positive and uniform results than 

 are obtained either with the Foucault form or the ordinary 

 spring contact form, but it seems to me to be considerably 

 more complicated than need be for the purpose. 



I am convinced from my own experience that mercury con- 

 tacts are unsatisfactory if the best results are to be obtained 

 with large induction coils. The electromotive force induced 

 in the secondary circuit depends on the sharpness of the 

 break in the primary, and this is lengthened considerably by 

 the oscillation of the mercury surface and the formation, (on 

 account of the low boiling point of the liquid), when the 

 break occurs, of a considerable amount of conducting vapor. 



* Wave Lengths of Electricity on Iron Wires. This Journal, vol. xlviii, p. 316,. 

 October, 1894. 



