a capacitive coupled clamp - on voltage probe having three concentric cylindrical capacitive plates each separated by a thin dielectric is adapted to be clamped onto an insulated wire such as the ignition wire in an automobile to measure the potential of the wire . in one embodiment the probe contains a solid dielectric center with a slot along its longitudinal axis which extends radially from the outside to the center of the probe thereby allowing the probe to be easily positioned over and released from an automobile ignition wire by slipping the probe over any insulated portion of the wire . the slot is dimensioned to hold the wire securely at the center of the probe . the probe also includes a releasable clamp having a thumb actuated lever to assist in maintaining the wire securely at the probe center and to permit easy positioning and release of the wire . in a second embodiment the probe is separated into two symmetrical semicylindrical halves which open and close by a spring loaded lever to admit the wire whose potential is to be measured .