Abstract: Apparatus for equalizing the electrically effective value of an electrically conductive layer is described wherein the equalization is effected by a tool, such as a sand-blasting device or laser that does not contact the layer, which is caused to traverse the layer to reduce the thickness thereof under the action of a thrust device acting on a pivoted tool guide under the control of an electrical circuit responsive the resistance of the layer. The tool guide is a two-part telescopic device pivoted about a point displaced above the working end of the tool by an amount that is many times greater than the distance of the working end of the tool above the layer, which distance is kept constant by controlling the length of the telescopic guide. For this purpose a feeler device on the lower part of the guide is maintained in contact with a cam surface which determines the length of the guide against spring reaction as the guide turns about the pivot point.