On applying electricity, always in the same direction, first to one and then to the other of the branches, it is observed that in both cases the needle tends to orient itself perpendicularly to the conductor, but turns in opposite directions. In the parallel connection, it is seen that the effect is almost null, while in the connection in series it is quite strong.
When the connection is in parallel, in fact, the current in one branch tends to make the needle rotate in one direction while the current in the other branch makes it turn in the opposite direction; when connected in series, the two currents give rise to a two-fold effect compared to when a single branch is electrified. In the latter case, the magnetic needle is at the centre of a spiral and the apparatus becomes, in principle, if not a true galvanometer, at least a "galvanoscope".