Hihi, I put a photodiode's legs to both the input for a differential amplifier http://en.wikipedia.org/wiki/File:Op-Amp_Differential_Amplifier.svg Then, the voltage difference of the photodiode is amplified and its output is then connected to a resistor which is not shown in the above picture. The purpose I put a resistor is to convert the voltage output to current. Then, I connect in series of the output from the resistor to an ammeter to obtain the current. After which, the other end of ammeter is connected to ground. May I ask, If I put one end of the ammeter to the ground and the other end to the resistor, will the system works, because from my high school knowledge of electric circuit, current flows only when a closed loop is formed, but I didn't see any closed loop here.