Outlets are not supposed to be dimmed!
While this might seem like a good idea if you wanted to plug in a table lamp that lacked its own dimmer, it creates a dangerous situation since outlets can accommodate any device with a plug, and some devices, such as many motors, react very badly to the output of a dimmer. It is forbidden under 404.14(E):
General use dimmer switches shall be used only to control permanently installed incandescent luminaires unless listed for the control of other loads and installed accordingly.
The other load dimmers are for fluorescent, LED or CLF fixtures. Dimmers are not listed for standard outlets.
The first thing I would do is replace the dimmer with a standard switch. (Done with power from the breaker turned off.) I would also check the outlet to see if it appears to be wired correctly -- black wire to brass, white wire to silver, green/bare to ground. Restore power and check the outlet again.
Supplement
Also see the answer by @ThreePhaseEel that explains specialized outlets that can be dimmed, and the associated code.
Unexpected voltage
Just to recap, a "phantom voltage" is where a wire from a switch to a light or outlet runs next to a permanently live wire, when the switch is off, the AC voltage in the other wire can induce a voltage in the switched-off wire. However, there is very little energy transferred so the phantom voltage is usually not dangerous because it can produce almost no current. However it can cause confusion and confusion can cause dangerous mistakes.
Most voltage testers and most multimeters are not useful for telling the difference between real and phantom voltages.
In When doing electrical work, what do I use to check wires are safe? I have a long answer that covers the different types of tester and some of their advantages and disadvantages - however it is very long and doesn't show how to use a tester.
Here's how I used mine when replacing a ceiling light:
First, with the breaker on and the switch on, I test that my voltage-tester is working.
Good, it shows 230 volts as expected.
Now I turn the switch off and check the voltage again. At this point you'd expect no voltage.
But no, it is reading 50 volts! what does this mean?
As explained above, this is a phantom voltage. How can we be sure?
The tester has a button marked "test". This puts a lower resistance across the live connection and allows a small current to flow (only about 3 thousandths of an amp)
When pressing the test button, the 50V dissapears. That shows that this is a phantom voltage with no real power behind it.
I can now turn off the breaker, check my tester again elsewhere and proceed to work on the wiring in safety.
UK wiring
Some notes about the wiring and light fittings shown above.
Don't worry if your wire-colours, connectors and light look nothing like this. The same principles apply wherever you are in the world, whether your voltage is 120 or 230 and no matter what colours your wires are.
If you are interested, read on, otherwise ignore what follows.
What is shown is a Swedish light fitting in a UK home. It uses a "2D" fluorescent bulb - which are no longer very popular and might not be found in your part of the world. However it works like any light bulb in that it lights up when mains/live/hot voltage is applied to it.
A common UK practice is to use the light fitting as a junction box for wiring fromthe supply, to and from the light-switch and onward to the next light fitting.
The three red wires on the right are
- live in from supply
- live out to switch
- live out to next light fitting
The black wire with the red-tape on it is a "switched live" coming back from the switch. This is what gets connected to one side of the light itself.
The two black wires are
- neutral in from supply
- neutral out to next light fitting
the green/yellow wires are three bare-copper ground wires which the electrician covered with a yellow-green sleeve.
- ground from supply
- ground to switch
- ground to next light fitting
In some countries you wont have any ground wires.
In some countries or some homes you won't have all the other wiring coming to the light fitting, you'll just have one one live/hot wire and one neutral wire. The colours depend on country. In the UK the colours have changed for European harmonisation. These are the old colours.
In other parts of the world, different types of connector are used. In the USA you will find "wire-nuts" twisted onto groups of wires.
Note that the connector blocks used in the UK allow you to fairly easily test voltages without undoing the connectors. The plastic insulation protrudes far enough from the metal inside that it is safe to handle the connectors with your fingers with wires in place. Howver there is room to get either an insulated screwdriver or a voltage-tester probe into the connector and touching the clamp-screw.
Best Answer
There is always voltage across the circuit, supplied by the power company. By Kirchhoffs's Voltage Law, the sum of the voltages across the individual components must always equal the supply voltage. When the light is on, there is little or no voltage across the switch and all of the voltage appears across the light. But when the light is off, all of the voltage appears across the switch and there is no voltage across the light (and no current through it).