a rolling shaft 31 a power 32 a zero device 33 and a controlling device 34 differentially an adjusting member 336 is disposed at one side of the zero device 33 for preferably adjusting the position of the sensor 334 further a governor 337 can be alternatively installed between the adjusting member 336 and the zero device 33 for controlling the output power of the adjusting member 336 accordingly by means of the additive adjustment member 336 cooperating with the governor 337 the position of the sensor 334 can be more delicately amended to collaborate with the actuating member 333 for achieving the merits of precisely controlling the covering sheet at the desired position obtaining an efficient speedy adjustment and attaining a convenient operation referring to fig9 a third preferred embodiment of the present invention comprises elements operations and functions similar to the first preferred embodiment in which the electric curtain 3 still comprises a rail 30 a rolling shaft 31 a power 32 a zero device 33 and a controlling device 34 specifically the arrangement of the interrelated elements in the zero device 33 can be either performed in the configuration like the first and second preferred embodiments or performed in the configuration like this embodiment namely the actuating member 333 of the zero device 33 in this embodiment is set as a discrete part to separate from the turning member 331 on which a plurality of apertures 332 are equidistantly defined and the sensor 334 is still correspondingly situated with respect to the turning member 331 accordingly when the turning member 331 and the rolling shaft 31 are concurrently rotated the sensor 334 would be able to sense the rotation of the turning member 331 and the actuating member 333 would be triggered by the.