beneath hole 22 and is slidable to permit covering and uncovering the deformable cover 32 cover 24 is relatively non deformable compared to cover 32 when cover 24 is covering deformable cover 32 cover 32 is prevented from deforming when cover 24 is not covering deformable cover 32 and when cover 32 is pushed by an operator 39 s finger cover 32 deforms substantially knob 26 is attached to and projects from cover 24 and may be grasped by an operator to slide cover 24 knob 26 fits into notches 28 and 30 in case 11 which define the limits to the sliding motion of cover 24 in fig1 cover 32 is uncovered and knob 26 is in notch 28 referring now to fig2 case 11 has an elongated cylindrical bore formed by the cooperation of channel 108 in base 16 and channel 110 in front cover 18 electrode 14 is mounted inside the bore formed by channels 108 and 110 which stabilize electrode 14 and restrict electrode 14 to lie along the central longitudinal axis 57 of the cylindrical bore formed by the cooperation of channel 108 and channel 110 the bore formed by channels 108 and 110 closely surrounds electrode 14 to prevent the entry of moisture or other foreign substances into case 11 electrode.