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XVIII. A new form of Amperemeter and Voltmeter with a 

 Long Scale. By Benjamin Da vies *. 



TT^HESE measuring-instruments, which are intended for 

 JL continuous currents, are of the so-called d'Arsonval 

 type, and their chief feature is their long and uniformly 

 divided scale. Mainly on account of these advantages, I am 

 of opinion that the instruments may become generally useful 

 for electrical measurement on switch-board or on the bench, 

 and particularly in the laboratory for purposes of experiment 

 and research. 



With the exception of instruments depending on magnifi- 

 cation, these (so far as I am aware) are the only long-range 

 ones yet designed ; a brief description of them may, there- 

 fore, not be unwelcome f. 



It is evident that u long scale is a desirable addition to the 

 best of instruments provided the extra length can be produced 

 ■without increasing the waste-power of the meter or without 

 reducing the deflecting moment. It is evident that this is 

 a question of design only, and the instruments described below 

 have a satisfactorily large deflecting moment, although the 

 scale covers an arc varying from 210° to 230°. By sacrificing 

 a little of the deflecting force, or by increasing the voltage 

 drop at the terminals of the moving coil, an instrument may 

 be constructed with a scale-length equal to an arc of 270°. 



The magnetic circuit also has one distinct point in its 

 favour, viz., its single air-gap. Tbis air-gap may be as 

 narrow as ^g of an inch, reducing the demagnetizing force 

 to a small quantity. 



The permanent magnet and the magnetic circuit generally 

 may have many forms, each having its own advantages and 

 disadvantages. The construction of the earliest form is 

 shown in fig. 1 . A is an iron case, either wrought or cast. 

 B is the soft iron polepiece, which is carried and maintained 

 in position by the permanent steel magnet M. 



From a physical point of view, this combination really 

 forms a very good magnetic circuit, for the reluctance of the 

 air-gap between polepiece and box is very small, owing to its 



* Communicated by the Physical Society : read Feb. 10, 1899. 



t After this paper was read I found 1 was by no means the first in the 

 field as regards " long range." Professor Ayrton kindly sent me a sketch 

 of an amperemeter by M. Carpentier, built on precisely the same principle 

 as those mentioned in this paper. The Carpentier instrument, however, 

 is not of good design, resulting, I should imagine, in an excessive drop of 

 voltage at the terminals of the moving coil. The Carpentier meter, I am 

 told, was brought out in 1889. The first form of mine was made in 1895. 



