Powering servos from an external power supply

Once buying some servos i simply used the sweep example to get the range of degrees. While testing this part i had simply just plugged it directly into the board of 5V. However i need the higher torque that comes with the higher voltage, 7V. So i hooked it up to a power supply i had. Upon doing this with the simple sweep program the servo failed to run, and every so often jerked about randomly. So i tried using a couple of different batteries but the same thing happened. I though maybe i messed up the servo in the process so i plunged it back into the board, it works perfectly there.
I do have a wall plug for the ardunio, and i do know about the vin pin that it is the power before going through the voltage regulator, for like a 9V battery. But i really don't want to be plugging in all the servos into the ardunio as with 4-5 servo being plugged in i believe it would just drain the power from other important parts of the board.
I don't know what i'm missing here, maybe i need a diode or a capacitor to regulate the power flow, or maybe its the servos themselves, which means this is never going to work.

Sometimes people forget to connect the Ground (Black) wire of the servo to both the negative side of the power supply AND the Ground pin of the Arduino. Without that Ground connection the Signal wire has no circuit.

Ah! that makes sense, i cant believe i forgot to simply complete the circuit. Thank you