on Galvanometers. 83 



complex result depending on many factors, it is desirable to 

 record the results of experiments on galvanometers in such a 

 form that the influence of each factor can be separately esti- 

 mated. We have therefore endeavoured to distinguish, as far 

 as possible, the three chief causes that affect the sensibility of 

 reflecting-galvanometers, viz. the arrangement and winding 

 of the coils, the construction of the magnetic system, and the 

 optical method of magnifying the angular deflexion produced. 

 With the same view the question of the susceptibility of 

 various galvanometers to outside magnetic disturbances has 

 been considered separately, since, first, the electrical qualities 

 of the instrument are not altered by such disturbances, and, 

 Secondly, their influence is dependent, not only on the kind of 

 instrument, but on the place of test. In the Table that is 

 given later on recording the results of a large number of 

 experiments, we have classified galvanometers in four divisions 

 of decreasing susceptibility to outside magnetic disturbance, 

 comprising : — 



iVbra-astatic instruments working in a weak field having 



strongly magnetized needles. 

 iVon-astatic instruments working in a strong field and 



having somewhat weakly magnetized needles. 

 Astatic galvanometers, working in a fairly strong field. 

 Galvanometers of the d'Arsonval type, working in an 



extremely strong field. 



At the end of the Table are given the constants of a collection 

 of non- reflecting instruments. These instruments, being all 

 of a rougher type, have been simply grouped together at the 

 end of the list ; consequently their position on the list has no 

 reference to their sensitiveness or unsensitiveness to outside 

 magnetic disturbance. 



The optical sensitiveness of an instrument is dependent on 

 many conditions that have no necessary connexion with the 

 electromagnetic qualities of the instrument ; for example, 

 the goodness of the mirror, the brightness of the light, the 

 distance of the scale, &c. are important factors. We have 

 therefore given the number of scale- divisions per micro- 

 ampere or per micro-coulomb, assuming the scale to be 

 placed at a distance from the mirror equal to 2000 scale- 

 divisions. As the scale-distance most generally used is 

 a metre, and as the scale-divisions are frequently in half- 

 millimetres, the conditions assumed are often fulfilled in 

 practice. The sensibilities for a steady current, and for a 

 discharge, are thus directly expressed in angular measure, 

 irrespective of optical magnification. 



G2 



