Measurement surface of the VE series Gap Detector consists of a measuring electrode, a guard electrode and a sensor case, which are in a three-layered structure as can be seen in the figure below. "The measuring electrode" is an electrode for measuring capacitance, and its diameter is less than 1/3 of the sensor’s diameter. (The size depends on the model.) "The guard electrode" is an electrode for ensuring the linearity of the sensor, and it prevents electric lines of force coming out of the measuring electrode from spreading out. In the following figure, red dotted lines represent electric lines of force coming out of the guard electrode. Because of this, electric lines of force coming from the measuring electrode (represented as blue dotted lines) can go straight to the conductor to be measured, and thus measurement of high linearity and high accuracy can be performed.