1881.] Prof. Helmholtz. On an Electrodynamic Balance. 39 



II. " On an Electro dynamic Balance." By H. Helmholtz, For. 

 Mem. R.S., Professor of Physics in the University of Berlin. 

 Eeceived April 7, 1881. 



In order to avoid the disturbances produced by the variations of 

 direction and intensity of terrestrial magnetism in measuring the in- 

 tensity of galvanic currents by their electromagnetic effects, I have 

 tried to construct an electrodynamic balance. I have suspended at 

 the ends of the lever of a smaller chemical balance, instead of the 

 scales, two coils of copper wire, their height being equal to the 

 diameter of the cylinder around which the wire is coiled up. Their 

 axis is vertical, and they are suspended in such a manner that they 

 cannot turn around this axis. Two larger spirals of the same height, 

 but of greater radius, are placed into a fixed position, borne by a 

 horizontal metallic rod, the middle of which is fixed on the column 

 bearing the balance. The connexions of the wires are arranged in 

 such a way that one of the movable coils is attracted by the fixed 

 coil, the other is repelled. Both the fixed coils are placed a little 

 higher than the movable coils. The attracted coil rises, the other 

 sinks down as soon as a current passes through the circuit. 



There are two difficulties to be overcome in the construction of such 

 a balance. At first, the current must be introduced into the movable 

 spirals without diminishing their mobility, and without introducing 

 places of contact of too small a pressure, which would make the 

 resistance variable. I have succeeded to do this in a very satisfactory 

 manner by using a kind of very thin sheet-brass,, used for playthings of 

 children, called in German " Rauschgold " (tinsel), because it looks like 

 gold, and makes a crackling noise when it is moved. Strips of this, 

 about 30 centims. long and 6 or 7 millims. broad, are very flexible, 

 and show no signs of internal friction, their resistance to electric 

 currents is very moderate, and they are not easily heated even by 

 strong currents, because they have a relatively large surface in con- 

 tact with air. I have connected each of the movable spirals with 

 the other wires conducting the current by two such strips hanging 

 loosely down from four pieces of brass fixed at the upper parts of the 

 case of the balance. I may be allowed to remark, that strips of the 

 same kind, and of greater length, are very useful to demonstrate the 

 action of a magnet on a movable current. If you suspend the strip 

 so that it hangs down in a curve, it is attracted, repelled, even 

 raised against gravity, or coiled up around the magnet with great 

 rapidity, in a very striking way. 



The second difficulty is to bring the coils into such a position that 

 neither the stability nor the sensibility of the balance is impaired. 

 In order to do this, it is necessary that the intensity of the electro- 

 dynamic force does not vary sensibly during the usual small oscillations 



