622 SCIENTIFIC RECORD FOR 1885. 



ance is a little more rapid than this mechaDical action. {J. Phys., April, 

 1885, II, IV, 171.) 



Topler has sug|!^ested the use of the balance in measuring the quan- 

 tity MH in determiniug the horizontal component of the earth's mag- 

 netism. For tbis purpose a magnetized bar is fixed perpendicularly to 

 the brass beam of a balance in place of the ordinary pointer. Weights 

 are placed in the scale pans in order to make the axis of the magnet 

 vertical when the plane of oscillation is in the magnetic meridian. The 

 balance is then turned through 180° and again the axis is adjusted to 

 verticality by shifting- the weights. MH is then equal to ^ (Qi— Q2) ^ ', 

 in which I is the length of that arm of the beam to which the weights 

 Qi and Q2 have been added. The author details the precautions neces- 

 sary to insure accuracy, and claims 40^0 ^s the limit of precision of the 

 method. ( Wied. Ann., xxi, 158; J. Phys., December, 1885, II, iv, 587.) 



Gray has described the methods in use in the laboratory of the Univer- 

 sity, Glasgow, for determining- the value of the earth's horizontal com- 

 ponent. Experiments to determine the effect of length and hardness 

 on the induction coefficient of the deflector magnets show that the 

 length of the magnets should be at least forty times their diameter and 

 that they should be made as hard as possible. The results are given 

 in a table. {Phil. Mag., December, 1885, V, xx, 484.) 



2. Electric Generators. 



The paper on the seat of the electro motive forces in the voltaic cell 

 with which Lodge opened the discussion of this subject at the Montreal 

 meeting- of the British Association has ai)peared in full, and is a most 

 valuable and impartial resume of the history of this important subject, 

 and an able argument in favor of the views that the apparent difference 

 of potential of copper and zinc is in fact the sum of a copper-air and a 

 zinc-air contact difference. {Proc. British Assoc, for 1884, 464; Proc. 

 aS'oc. Teleg. Eng. and Elec, xiv, 186 ; Phil. Mag., March, April, May, 

 June, October, 1885, Y, xix, 153, 254, 340, 487; xx, 372.) 



Koosen has constructed a form of battery based on the depolarizing 

 action of bromine. It consists of a glass jar having a narrow prolonga- 

 tion at its lower portion to contain the bromine, above which is a porous 

 plate supporting- a porous cup containing a rod of amalgamated zinc. 

 A platinum wire traversing the porous plate makes communication 

 with the bromine. The uijper portion of the jar is tilled with dilute 

 sulphuric acid, on which rests a thin layer of jjetroleum to prevent the 

 escape of the bromine vapors into the atmosphere. The electro-motive 

 force of the combination is 14) volts and becomes somewhat higher if 

 the platinum is replaced by carbon. The internal resistance is consid- 

 erable, but the battery is very constant. {Wied. Ann., xxiii, 348; J. 

 Phys., August, 1885, II, iv, 373.) 



Von Helmholtz has constructed a modified form of Daniell cell, which 

 consists of a deep glass goblet, in the bottom of which is a copper spiral 



