What you have stumbled upon is a voltage divider. Essentially, the voltmeter you are using is putting a very high resistance across the copper leads, and measuring the voltage with the meter gave a reading of 40.
Now since you have said that this is an old house, I am assuming it is built before the 1950s and may or may not have 'old' wiring inside.
Since you say it is 40 Volts ac, I am assuming it was a traditional meter with a reflective back, and a red pointer to indicate what the reading is. If it is a Digital Multimeter, then the same principles apply, just a slightly different reading is what is needed.
In Electric Circuit, there must always be a loop, a place where the electricity starts, a current path, a load (or light or motor), then finally a return for the electricity to go after it has been used up.
With AC power found in most homes, either in North America and Japan with 120 Volt Potentials, or with the rest of the world with 240 volts, the principle is still the same, there is one source wire, a switch, then a lamp or outlet, and finally a return wire.
Now you also mentioned that the wires were copper, and you are trying to replaace a two button toggle swich with a more modern looking type. I am assuming that the switch turns on a light of either a low power LED or similar smlll current device (say 2 milli-ampers).
The Meter was actually reading the voltage developed across the switch contacts, (or was a dimmer set for 40 volts), and was reading across the switch terminals.
If the Meter was drawing 2ma, and it's sensitivity is 20,000 ohms per volt, then to have the meter fully deflect (jump to the right), that would mean it would have to have a voltage divider, with the meter conducting 2 micro amps of current through the coil of the movement.
those two microamps are what were been read by the movement, because the rest of the current , 2 milli-amps were going through the meter when the 'voltage ' setting was being read, and it was mot enough current to light up the lamp in the fixture.
THe lamp in the fixture needs 25 milli-amps to even glow, or have the LED's start to conduct, not the 2 milli-amps that were flowing when the meter leads were connected across the switch terminals.
If you close the swich, and measure the voltage across the two terminals (safety first) then the actual voltage will read ZERO.
However, if you measure one of those copper wires, and a reference ground then you will read 120 volts(240 for the rest of the world). The other wire will go to the lamp or whatever is being driven by the circuit.
shut off the power, by either throwing the circuit breaker for that branch or removing the fuse for that part of the house, then replace the older switch with a new one.
Oh, and when the reading was made , never set the meter to Amperes and insert the leads into any voltage source. This is a good way to blow up the meter, and cause harm to you or the wires.
the only way, other than the dimmer being in series, is if the circuit is passing such a small current, that the meter 'read' incorrectly, since the whole home is wired for 120 volts, the '40' reading was the leakage of the current through the meter.
wikipedia article on circuits