You can have output AC voltage that can be lower or higher than the input AC voltage.

I have a rheostat.

That is not the determining factor. The determining factor is what kind of material is the core made of.
If it is ceramic then it is a rheostat.
If it is ferromagnetic then it is an autotransformer. The ferromagnetic material would make the autotransformer much heavier than a rheostat of similar size.

 

Well, I just edited my previous comment to state that what I have is an autotransformer. It has a ferrous core. Magnet proved it.

 

The fundamental property that defines an auto-transformer and makes it different from transformer with multiple windings connected to one terminal, is that in an auto-transformer both primary and secondary currents pass through the primary winding. That comes from a first-year basic electricity class, in the AC section, which follows the DC section and very basic circuit theory. So just because it has the windings tied does not make it an auto-transformer, although that is also a requirement. Nessesary but not sufficient.

 

It's a rat zapper. The primary is a driven voltage - whatever that may be; and the secondary delivers a lethal shock to the animal. Likely its primary winding is heavier gauge and the secondary is likely a very fine wire with many many turns to get the charge sufficient to kill the vermin. Likely. Not definitely. Perhaps an ohms reading might shed some light.

 

It's a rat zapper. The primary is a driven voltage - whatever that may be; and the secondary delivers a lethal shock to the animal. Likely its primary winding is heavier gauge and the secondary is likely a very fine wire with many many turns to get the charge sufficient to kill the vermin. Likely. Not definitely. Perhaps an ohms reading might shed some light.

No. An ohms reading is not particularly helpful.
The resistance measured is proportional to the length of wire x resistivity and inversely proportional to the square of the diameter of the wire.
The resistance gives an indication of the current capacity of the winding, not the voltage or whether it is primary or secondary.

Edit:
Put it this another way. The manufacturer of a coil of wire will use the smallest diameter wire for the current required in order to keep costs down.
A high voltage, low current coil will demand a large number of turns of small diameter wire. This will therefore have a high resistance.
A high voltage, high current coil will use the same number of turns of larger diameter wire. The resistance will be lower.

 

No. An ohms reading is not particularly helpful.
The resistance measured is proportional to the length of wire x resistivity and inversely proportional to the square of the diameter of the wire.
The resistance gives an indication of the current capacity of the winding, not the voltage or whether it is primary or secondary.

Edit:
Put it this another way. The manufacturer of a coil of wire will use the smallest diameter wire for the current required in order to keep costs down.
A high voltage, low current coil will demand a large number of turns of small diameter wire. This will therefore have a high resistance.
A high voltage, high current coil will use the same number of turns of larger diameter wire. The resistance will be lower.

At some point the cost of thinner wire increases, and the production yield drops. BUT an ohm meter check will certainly reveal the end of the secondary. Probably it will take a magical meter to tell the end of the primary from the common. it will be easier with a magnet and a compass.