Highly conductive, these loops come with adjusters to allow tightening to your personal preference.  Just push on the button, tighten and release. An added benefit is these can double as cock rings to allow longer, more consistent erections. The inner diameter is 3.75mm so any banana plug will fit snugly.  

Order one, a pair, or a variety with one of each. Available with single or double cord locks. (in photos, singles are on left, doubles are on right) Single cord locks put both end pieces of the loop through one hole which gives a completely surrounded, stimulation. Double cord locks use two holes so have a slight break where the plastic is between the holes. 

Be sure to use water-based lubricant such as Spectragel to keep the sensations smooth and avoid hot spots. Never use any lube with silicone of any kind- it weakens and degrades the rubber.