In the first case the direction of the current in the wire and the filament changes simultaneously. Therefore, depending on how the ends of the filament are connected to the grid, the direction of the currents in wire and filament at any moment will either be the same, in which case the filament will be attracted to the wire, or opposite, in which case the filament will be repelled. In the second case the direction of currents will be the same for one half-period and opposite for the following half-period. Therefore the filament will be alternately attracted to the wire and repelled, i.e. it will perform oscillations about the position of equilibrium; the frequency of its oscillations will be equal to the frequency of the town-grid's a.c. current.