OBJECT OF THE MEMOIR. 73 



CHAPTER VIII. 



ON THE USE OF A SECONDARY WIRE AS A MEASURE OF THE RELATIVE TENSION OF 



ELECTRIC CURRENTS. 



(From the London and Edinburgh Philosophical Magazine for October and November, 1839.) 



CONTENTS : Object of the Memoir. Action of a Secondary Wire. Description of the 

 Torsion Galvanometer. Resistance of the Secondary Wire under Variations of Ten- 

 sion. Condition of the Current never changes. Tension rises with Length of Wire 

 and with Distance of Plates. Relation between Quantity and Tension. Theory of 

 Tension of the Voltaic Battery. Known Methods of increasing Tension of Currents. 

 General Law. Caseof Thermo- Electricity and Machine Electricity. Voltaic Spark 

 before Contact in Vacua. 



240. IT is the object of this memoir to establish the following propositions : 



1st. That by means of a secondary wire, we may always determine the relative ten- 

 sion of electric currents. 



2d. That there is reason to doubt whether the processes usually supposed to affect 

 the condition of an electric current are ever attended with any such result ; but that, 

 when changes have apparently taken place, it is probable that they may be directly 

 traced either to a disturbance at the place of generation, or to the development of other 

 currents of a different character, the primary current itself remaining unchanged. 



3d. That there are two different methods of accomplishing these disturbances, and 

 thereby of raising the elastic force of a current : 1st. That tension may be augmented 

 by the sacrifice of quantity ; Volta's plan of a reduplicated series, and Henry's riband 

 coil, in its condition of equilibrium, being examples : 2d. By the introduction of new 

 affinities in the exciting cells ; batteries charged with nitrosulphuric acid or sulphate of 

 copper are examples. 



4th. That the law which regulates the connexion of this diminution of quantity or 

 condensation with the increase of tension is the same as that which regulates the anal- 

 ogous phenomena of ponderable elastic fluids. 



241. Incidentally, the examination of certain other points will be entered upon: for 

 example, a brief consideration of Lenz's law of the conducting power of wires ; this, it 

 will be shown, holds not only in the case of Faradian currents, but in the direct cur- 

 rents from hydro-electric and thermo-electric pairs, as has been advanced by some 

 philosophers, but denied by others. 



242. The terms tension, intensity, tensile effect, &c., have had very different signifi- 

 cations attached to them. From this circumstance a great deal of confusion has arisen, 

 and it is one of the causes of that diversity of opinion and contrariety of theory which 

 obtain in the elementary parts of the science of electricity. For example, DR. FARADAY 

 appears to use the words tension and intensity as synonymous, expressive, as it were, of 



K 



