Prof. Wheatstone on Voltaic Instruments. 381 



indicate the existence anywhere in the southern hemisphere of a 

 higher intensity than would be expressed by 2*1 of the arbitrary 

 scale. In this respect also the analogy between the two hemi- 

 spheres appears to be closer than is shown in M. Gauss's maps, 

 Plate XVIII. With respect to the direction of as much of the line 

 of highest intensity (2*0) as it has been possible to draw with any 

 degree of confidence from the observations now communicated, it 

 will be found to be in almost exact parallelism with the isodynamic 

 line of 1*7 in Plate III. of the author's report " On the Variations of 

 the Magnetic Intensity," in the Report of the eighth meeting of the 

 British Association, for ] 838 ; which line was the highest of which 

 the position could be assigned at that period for any considerable 

 distance by the aid of the then existing determinations. 



3. " An Account of several new Instruments and Processes for 

 determining the Constants of a Voltaic Circuit," by Charles Wheat- 

 stone, V.P.R.S., Professor of Experimental Philosophy in King's 

 College, London, Corresponding Member of the Royal Academy of 

 Sciences at Paris, &c. 



The author proposes in the present communication to give an ac- 

 count of various instruments and processes which he has employed 

 during several years past for the purpose of investigating the laws 

 of electric currents. He states that the practical object for which 

 these instruments were originally constructed, was to ascertain the 

 most advantageous conditions for the production of electric effects 

 through circuits of great extent, in order to determine the practica- 

 bility of communicating signals by means of electric currents to more 

 considerable distances than had hitherto been attempted. Their use, 

 however, is not limited to this special object, but extends equally to 

 all inquiries relating to the laws of electric currents and to every 

 practical application of this wonderful agent. 



As the instruments and processes described by the author are all 

 founded on Ohm's theory of the voltaic circuit, he commences with 

 a short account of the principal results to which this theory leads, 

 and shows how the clear ideas of electromotive forces and resist- 

 ances, substituted for the vague notions of intensity and quantity 

 which formerly prevailed, furnish us with satisfactory explanations 

 of phenomena, the laws of which have hitherto been involved in ob- 

 scurity and doubt. According to Ohm's system, the force of the 

 current is equal to the sum of the electromotive forces divided by 

 the sum of the resistances in the circuit. The several electromotive 

 forces and resistances which enter into the circuit of a voltaic battery 

 are then defined ; and having frequent occasion to refer to the laws 

 of the distribution of the electric current in the various parts of a 

 circuit, when a branch conductor is placed so as to divert a portion 

 of the current from a limited extent of that circuit, the author directs 

 particular attention to these laws. After recommending several 

 new terms in order to express general propositions, without circum- 

 locution and with greater precision, the author states the method of 

 obtaining the constants of a circuit employed by Fechner, Lenz, 

 Pouillet, &c, and then proceeds to explain the new method he has 



