Transcutaneous electrical nerve stimulation (TENS) characteristic for safe cat/dog static shocker

Short answer:

Don't ever administer electrical shocks to pets because you cannot control how well the current is conducted through the fur.

The following information is my personal experience as a medical device developer and former user of TENS for pain relief.

The intensity of TENS stimulation depends on several different factors, like:

• The frequency and strength of the stimulation. Low frequencies tend to cause muscle contractions, while high frequencies can block the transmission of nerve signals. This is used in body shaping applications to exercise muscles and in pain therapy to block pain signals. Low strength causes no sensation at all, while high strength always causes pain, no matter the frequency. High strength can also physically burn the skin.
• The condictivity of the skin: The moisture content at a given time (for example dry vs. sweaty skin) makes a notable difference in the sensation of the stimulation. Dry skin feels slightly more painful than moist skin. Wet skin (as could happen to a cat in rain) can create a short circuit and completely nullify any stimulation.
• The size of the electrodes: Big electrodes (a square inch and more) are most often used for pain relief, because they spread the current over a larger area and seem to lead it deeper into the body. Small electrodes concentrate the current to small spots on the surface on the body. I once had an electrode loosen during TENS pain therapy and half of it not sticking to my skin anymore. The sensation went from a comfortable buzzing to a painful pinch in a second. Imagine what those spiky little electrodes would feel like!
• The hairyness: Hairs are actually electrical resistors. All of the TENS devices for humans I ever saw instruct you to shave the site of application if it's too hairy. (Side note: mobile automatic defibrillators always contain a disposable razor because body hair impacts the conductivity too much). Applying an electrode to hairy skin causes intense burning pain right at the hair follicle! It would probably physically burn the follicle, but I wasn't willing to conduct this particular experiment.
• The site of application: TENS electrodes should always be applied to muscles and meaty parts of the body. Applying them directly to joints, where the skin tents to be thin and there are no muscles to conduct the current, can lead to pain and actually burning the skin.

Because of all these factors influencing the perception of an individual, there isn't even a baseline voltage / amperage for humans. There are special medical devices that are intended to cause a patient pain by TENS current to objectively measure the intensity of their chronic pain, but what doctors measure is never the absolute amperage, but rather the difference between the minimal amperage that is perceivable to this individual patient at all and the amperage that causes the same amount of pain as their chronic ailment.

So how could anyone ever determine the amperage of such a shock collar for your cat without asking the cat how much pain it feels while administering electricity?

And lastly the simple fact that a collar always sits around the neck and that those heavy electrodes always sit at the throat poses another problem many pet owners willingly ignore:

All instruction manuals for TENS units for human applications warn against placing the electrodes at your neck (either front / throat or letting the current run through your spinal cord). So why is it OK for us to place TENS electrodes at the throats of our pets? Wikipedia states:

There are several anatomical locations where TENS electrodes are contraindicated:

• On the front of the neck due to the risk of an acute hypotension (through a vasovagal response) or even a laryngospasm

TENS stimulation at the throat can lead to muscle spasms, loss of conciousness due to acute drop in blood pressure, choking, respiratory distress or accidental inhalation of things like saliva, which in turn can cause pneumonia.

Proposal for an alternative: It seems counter-intuitive for us humans, who always have a phone with vibration close by, but a vibration can be as effective in those shock collars as an electrical shock. All you need is to connect a certain action (like walking outside your yard) with a negative result. A startling tugging and vibrating collar is negative enough for most pets, you don't need to electrically shock them. Pets are very sensitive at their necks and may interpret a sudden vibration as being attacked or seized at their throat. That's shocking enough to discourage the initial behavior.