Using an NMOS as a switch for a 10V DC source in a 555 timer circuit
I didn't have time to answer before, but as I said in my comment the FET must go in series with the supply and the load (as with any switch).  This is shown below.
In the diagram on the left it shows an N-FET switch, but note the need to drive the gate well above the 10V supply to turn it on.  (You want the source at 10V, so the gate to source voltage has to be well above the threshold voltage, by enough to achieve the desired RDSon.  The diagram shows 15V as an example, but that may change depending on the FET.
If you use a P-FET as in the diagram on the right, you can turn it on by applying 0V to the gate, and off with 10V.  Therefore you don't need the additional supply voltage.  You have to check to be sure that the max Vgs is less than -10V.



simulate this circuit &ndash; Schematic created using CircuitLab