

used in Electrical Measuring-Instruments, 437 



moment of inertia. From this last column it may be seen 

 that the ordinary Siemens dynamometer type of coil, and those 

 of d'Arsonval's galvanometers and wattmeters, are far from 

 being the most efficacious. 



The numbers in column 4 are obtained on the assumption 

 that the coils are long in proportion to their breadth ; and 

 when this condition is not fulfilled (as it very seldom is), the 

 numbers would be still further reduced, for the parts of the 

 wire at right angles to the axis of suspension add to the 

 moment of inertia but not much to the deflecting force. 



This consideration is of importance where the coils are of 

 sections such as (4), (7), (8), (9) ; for as the coils are wide, 

 the useless moment of inertia sometimes amounts to one -fifth 

 the useful. In such a case as (8) the real numbers represent- 

 ing the efficacy of the coil, as compared with one made to 

 section (1) of the same moment of inertia, would be about 1 

 to 3. These latter numbers represent approximately the 

 efficacy of the coil of an ordinary Siemens dynamometer 

 reading up to 60 amperes, as compared with that of a coil 

 made to the best shape. It is thus possible to increase the 

 deflecting moment for a given current threefold by changing 

 the shape of the movable coil and arranging the fixed coils to 

 produce the same strength of field as at present used. 



Of course the new coil would be heavier than the old one, 

 and would therefore require a stronger suspension which 

 might introduce more vagueness of zero ; but as the friction 

 at the mercury-cups is considerable, the necessity for using a 

 stronger suspension would not seriously interfere with the 

 accuracy of the instrument. 



It will be noted that in the preceding part of this paper the 

 moment of inertia of the suspended coil has been assumed 

 constant ; it is perhaps desirable to give reasons why this 

 assumption is made. 



In most instruments having movable coils which are in 

 ordinary use, the current is led into and from the coil either 

 by mercury-cups, or by wires which also serve as the control, 

 or by flexible wires independent of the control. 



Taking the first case of mercury-cups, it may be noted that, 

 owing to friction and viscosity, a certain minimum control 

 is necessary to give the requisite definiteness of zero. As 

 time is generally of importance in making commercial mea- 

 surements, the period, of oscillation may be taken as constant. 

 Therefore, as both control and period are determined by 

 circumstances other than those affecting the shape of the coil, 

 the moment of inertia may be taken as constant. 



When the current is led into the coil bv the torsion-wires. 





