Hair Trimmer
The subject here is the cord type (plug-in) electric hair trimmer that you buy to give your family haircuts at home.  It probably applies equally to the products intended for giving your pet a trim as well; as far as I can tell the products are similar, the main difference being in the attachments.  I don't address the cordless units, although in general I'd suggest using a cord type unless you really need that much portability for some reason.  I'm also not addressing nose- and ear-hair trimmers or other specialty trimmers.

I don't claim to be an authority on hair trimmers; I'm just a dissatisfied customer who bought several lousy ones before I figured out what to look for.  I'm hoping to save others some of this grief.

All hair trimmers have similar cutting operation:  A stationary blade with a row of teeth is drawn through the hair while another blade with a similar row of teeth oscillates side to side across it.  The moving teeth sliding across the stationary teeth act as a dozen little pairs of scissors, neatly snipping each hair that they encounter.  All but the cheapest trimmers have a lever on one side that allows you to relocate the moving blade forward or rearward a little, which affects how closely the trimmer will cut.

Within the body of the trimmer is the mechanism that drives the moving blade.  There are two types, which for lack of better nomenclature I shall call "vibrating" and "motorized".

In the vibrating type, a stationary electromagnet is positioned close to a small block of iron on a flexible mount.  The cyclic nature of the 60Hz electricity from the wall outlet causes this block of iron to vibrate on its mount.  A linkage attaches this vibrating chunk of metal directly to the moving blade.  This type of trimmer has what looks like a plastic slotted screw head in the side of the body.  The instructions say to adjust this screw by turning it counterclockwise until the trimmer gets really noisy, and then back to the clockwise direction just until the racket drops off and it resumes a regular buzzing sound.  The kit includes a plastic screwdriver on the back end of the little brush for cleaning the cutters.

Not to put too fine a point on it, these types of trimmers aren't worth a crap.  They rely on resonance to generate enough travel in the moving blade -- that's what you're adjusting with the screwdriver.  But the resonance changes as soon as the blade encounters hair!  90% of the time, the moving blade is barely moving, not far enough for good scissor action, and you end up pushing the trimmer over the same place several times wondering if it'll ever cut.  Do yourself a favor and return that POS to wherever you bought it and demand your money back.

The good trimmers actually have an electric motor inside, meaning something with bearings and a rotating shaft.  The rotating shaft typically turns an eccentric that drives the blade back and forth.  As long as the motor is running, that blade will make a full sweep back and forth, neatly scissoring hair.  Even if you manage to load the motor down so it was only running at half speed, it would still be taking a full cut with each movement of the blade.

Atrociously expensive?  Apparently not!  I found a very nice one at Walmart for about $20.  It's made by Andis.  Walmart actually carries several models of Andis "home haircutting kit", but as far as I can tell they all use the same motorized trimmer.  The difference lies in the attachments and accessories -- and in the paint job, one of the more expensive ones looks like it's gold plated!

Now, while you're in Walmart, notice the Conair models.  Right on the box it says "electric motor" and has a little illustration of what clearly is intended to convey a rotating motor -- but it lies, the Conair is a vibrating type!  Blatantly false advertising.  The dead giveaway is that screw adjustment on the side.  The Andis doesn't have one; there's nothing to adjust on an electric motor.  If the trimmer you're looking at has that slotted adjuster on the side, my advice is don't buy it.  Period.  I don't care what company makes it or how much it costs.