on Galvanometers. 87 



2000 scale-divisions for a periodic time of 10 seconds that 

 would be obtained if all the different forms of galvanometers 

 were wound with such a gauge of wire that the resistance of 

 each instrument was 1 ohm. Column 11 gives the swing per 

 micro-coulomb on the same hypothesis, but in this case we 



have only used r z as the divisor. 



And whereas columns 9, 10 ; andll give the relative values 

 for the same periodic time, columns 12 and 13 give the 

 deflexion per micro-ampere and the swing per micro-coulomb 

 on a scale whose distance is 2000 scale-divisions for the same 

 controlling -moment that would be obtained if all the galvano- 

 meters had 1 ohm resistance — the periodic time in any case 

 being equal to 10 s/ 1 seconds, I being the moment of inertia 

 of the suspended system given in column 6* 



Another question to be considered is the factor of merit in 

 connexion with the volume of the coils. As this is important 

 as regards the cost of high-resistance galvanometers, a separate 

 column (5) is therefore given, showing the approximate volume 

 in cubic centimetres occupied by the wire in each instrument, 

 and other columns (14, 15, 16, 17) give the values obtained 

 by dividing this quantity into the various factors of merit. 

 The results of comparison in these latter columns are very 

 interesting. It appears that galvanometers of the Kosenthal 

 or Gray and the dArsonval types are by far the most sensitive 

 in proportion to the volume of coil ; and we believe that the 

 best way to make a very sensitive galvanometer with a movable 

 magnetic system is to employ several small coils, instead of 

 one or two large ones*, and that the magnets should be horse- 

 shoe-shaped, with the line joining the poles vertical, as in 

 Prof. Gray's instrument, but modified so as to give smaller 

 moment of inertia. Such a magnetic system can be made very 

 delicately astatic, and the weakening of the magnets with time 

 does not greatly influence the astaticism. We also conclude 

 from our investigations that the most sensitive galvanometer 

 of all would be one of the d'Arsonval type suitably modified. 

 The poles of the magnet should be very close together, the coil 

 should be very long and narrow, and no stationary iron core 

 should be used inside the coil as in the ordinary d'Arsonval 

 galvanometer. The sensitiveness could be further increased by 

 employing electro-magnets instead of permanent ones to pro- 

 duce the deflecting field, the current flowing round the electro- 

 magnet being kept constant by means of an auxiliary reflecting 



* Mr. C. V. Boys, F.R.S., in his Cantor Lectures on "Instruments for 

 Measuring Eadiant Heat" (April 1889), has also shown that galvano- 

 meters with small coils may be made very sensitive(pp. 22, 23). 



