182 Prof. A. Schuster's Electrical Notes. 



the Wheatstone-bridge over any other halance method. We 

 obtain for all of them, on the contrary, the same limiting 

 value. When a differential galvanometer is used, the two 

 opposing currents pass through the coils of the instrument. 

 Whenever there is any danger of disturbances in the indica- 

 tions of the needle due to heating-effects within the galvano- 

 meter, the Wheatstone-bridge will have the advantage. I do 

 not know whether there are any actual measurements as to 

 the strongest currents we can send through a mirror-galvano- 

 meter ; but it seems likely that when the resistances to be 

 measured are such that weak currents only can be used, as 

 with a platinum thermometer or bolometer, the differential 

 arrangement will possess many advantages. 



It appears that, in order to obtain the best effects from a 

 Wheatstone-bridge, the resistance to be measured must adjoin 

 one which is small and one which is great compared to itself; 

 and these resistances must of course be constructed so that 

 they are not overheated when the maximum current passes 

 through the resistance which is to be measured. This we may 

 not be able always to accomplish easily, and in practice the 

 sensibility of the Wheatstone-bridge may have to be reduced 

 still further. I think, therefore, that for measurements of 

 resistance in which a high degree of accuracy is required, 

 the differential galvanometer deserves more attention than it 

 has so far received. If galvanometers can be constructed 

 so that convection-currents due to heat-effects in the cell 

 containing the suspended magnets can be avoided, it is very 

 probable that the differential galvanometer will often prove 

 to be the most suitable instrument. 



With respect to the maximum current which we may be 

 capable of sending through galvanometers of the same type, 

 it is to be noted that the magnetic field at the centre of the 

 coils is proportional to the square root of the electric work 

 done in the instrument. When the heating-effect has reached 

 the maximum allowable limit, the magnetic fields in different 

 instruments having similar coils, but wound with different 

 wires, will be the same. 



The proposition embodied in equation (4) may give some 

 useful hints in the construction of bolometers or platinum 

 thermometers. Thus we may ask the question whether for 

 certain purposes some other metal may with advantage be 

 substituted for platinum. 



The rise in temperature of a conductor St is connected with 

 the electric work (W) by the equation 



MsSt=W, 



