﻿558 Mr. Carl Barus on Telephonic 



one tenth as heavy with a 10 sec. period. With very light 

 systems the mirror is unduly heavy in proportion to the 

 weight of the magnet systems, and the difficulty of handling, 

 and especially of astaticizing, is much increased. Their 

 principal disadvantage, however, is their extreme sensitiveness 

 to vibration. A system weighing 25 mgs. was first used in 

 the galvanometer described above, but it was found abso- 

 lutely unfit for photographic work because of the unsteadiness 

 of the image. 



Indeed, even with the heavier system, the complete elimin- 

 ation of the effects of vibration proved a troublesome problem, 

 for the Observatory is in close proximity to several traffic- 

 laden streets, and the work had necessarily, because of its 

 nature, to be carried on during the busiest part of the day. 

 Very good results were finally obtained by use of the insulated 

 fibre support, already described, in conjunction with the use 

 of rubber blocks between the slab of stone on which the 

 galvanometer was mounted and the pier which supported the 

 whole. Under very favourable circumstances a somewhat 

 lighter system than the one here recommended could no 

 doubt be used ; but in the great majority of cases I believe 

 that the weight should not be less than 30 to 40 mgs., and 

 may with advantage be considerably more. 



Astro-Physical Observatory, 

 Washington, June 1893. 



LXV. Telephonic Measurement of Electromotive Force, 

 By Carl Barus*. 



1. nURPOSE. — Notwithstanding the varied use which 

 has been made of the telephone in electrical measure- 

 ment, I am only aware of isolated efforts f to replace the 

 galvanometer by the telephone in the zero methods for elec- 

 tromotive force. Yet, according to the earlier observers, the 

 telephone ought to be more than sufficient for the purpose 

 (§ 2). The problem suggested itself to me in connexion with 

 thermoelectric pyrometry, where an avoidance of the galvano- 

 meter would often facilitate the work. Recently I thought 

 of it again in relation to certain meteorological experiments, 

 in which temperatures are made to vary in rapid rhythm by 

 condensation, and the object is to find the thermal amplitude 

 and the character of the oscillation. It thus becomes neces- 

 sary to vary the electric contacts in like rhythm and to find 



* Communicated by the Author. 



f Ledeboer, Beibliitter, ix. p. 357 (1885). 



