308 
MR. WHEATSTONE ON NEW INSTRUMENTS AND PROCESSES FOR 
general employment. Fechner # measured the force of the current by the number 
of oscillations of the needle when placed at right angles to the coils, a very tedious 
operation ; and others have employed the deviations of the needle, the corresponding 
degrees of force having been previously determined by some peculiar process, or 
inferred from some rule depending on the particular construction of the instrument. 
Another impediment to the use of a galvanometer to measure the force of a current 
arises from the changes in the magnetic intensity of the needle which frequently 
occur, especially when it has been acted upon by too strong a current. 
The principle of my method is that of employing variable instead of constant re- 
sistances, bringing thereby the currents in the circuits compared to equality, and in- 
ferring from the amount of the resistance measured out between two deviations of 
the needle, the electro-motive forces and resistances of the circuit according to the 
particular conditions of the experiment. This method requires no knowledge of the 
forces corresponding to different deviations of the needle. 
To apply this principle it is requisite to have a means of varying the interposed re- 
sistance so that it may be gradually changed within any required limits. I have 
contrived two instruments for effecting this purpose, one intended for circuits in 
which the resistance is considerable, the other for circuits where the resistance is 
small -j~. 
* Massbestimmungen iiber die Galvanische Kette. Leipzig, 1831, p. 5. 
f It appears that the idea of constructing an instrument of this kind had also occurred to Professor Jacobi 
of St. Petersburgh. When I explained to this eminent experimentalist my instruments and processes in the 
beginning of August 1840, he informed me that he had himself constructed a similar instrument which he had 
exhibited to the Academy of Sciences at St. Petersburgh, though no description of it had yet been published, 
and he at the same time showed me a drawing of it. This instrument, which he has since called an Agometer, 
differs in mechanical construction from either of mine, and is less convenient to manipulate ; but its principle 
is the same. In a communication which Professor Jacobi made in the following month to the Meeting of the 
British Association at Glasgow, and which was published in the Athenaeum of No. 678, 1840, he thus alludes 
to the subject : — 
“ Before proceeding, I maybe permitted to make some remarks concerning an instrument which I laid before 
the Academy of Sciences in the commencement of this year. It is destined to regulate the galvanic current, 
and is of value in many investigations of this kind. During my sojourn in London, Professor Wheatstone 
has shown me an instrument, founded exactly on the same principles as mine, and with very insignificant modi- 
fications and differences. Now, it is quite impossible that he should have had the least notice of my instru- 
ment ; but as it is probable that its use may be greatly extended, I must add, that while I have only used this 
instrument for regulating the force of the currents, he has founded upon it a new method of measuring these 
currents, and of determining the different elements or constants which enter into the analytical expressions, 
and on which depends the action of any galvanic combination. It is principally to the measure of the electro- 
motive force, by those means, that Mr. Wheatstone has directed his attention; and he has shown me, in his 
unpublished papers, very valuable results which he has obtained by this method.” 
Professor Jacobi has since his return employed my method of determining the constants of a voltaic circuit. 
The memoirs in which his results were given were republished in Poggendorff’s ‘ Annalen der Physik, vol. liv. 
No. 2. for 1841, and vol. Ixii. No. 9. for 1842. To the latter the learned editor, w r ho has made most valuable 
researches himself in the same path, has appended (p. 89) the following note: — “I will take this opportunity 
to call to mind that I applied the same method (or at least one identical to it in principle) before it was com- 
