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• The potential energy of a charged particle in an electric field is proportional to its charge - the electrical potential difference between two points in the field of which a potential energy of 1 C charge changes by 1 Joule, is defined as 1 volt (V).
• 1 Amp represents the transfer of 1 coulomb of charge per second
• All materials offer resistance to the flow of electrons
• Based on Ohm’s Law, the flow of a current of 1 Amp through a material of resistance of R ohms () produces a drop in electrical potential or a voltage difference of E volts across the resistance such that E=IR
AC and DC current• DC - direct current - the polarity of a current source remains
the same when the current is DC• AC - Alternative current - this is generated by using a
magnetic field (generator) to convert mechanical into electrical energy - the polarity changes with motion
• AC is characterized by its frequency (f) measured in hertz (Hz) (cycles per second)
• The AC output of a generator is frequently in the form of a sine wave - thus the voltage V(t) at any given time (t) is related to the maximum voltage Vmax by
Capacitance• A capacitor is a device with 2 conductors separated by an insulator
• If a DC current is applied to a capacitor a transient current flows but stops when the potential difference between the conductors equals the potential of the source
• If the source is removed the charge remains and can be release as current
• The capacitance measured in Farads (F) is equal to the amount of charge on either electrode in Coulombs divided by the potential difference between the electrodes in volts - 1 Farad = 1 coulomb/volt
• DC current will not flow “through” a capacitor - AC current will and the higher the frequency the better the conduction
The Coulter Principle•2 chambers filled with a conductive saline fluid are separated by a small orifice (100m or less)
•Thus, most of the resistance or impedance is now in the orifice.
•By connecting a constant DC current between 2 electrodes (one in each chamber), the impedance remains constant. If a cell passes through the orifice, it displaces an equivalent volume of saline and so increases the impedance.
detection• Since the analog pulse is very short (microseconds
typically) the only way to retain this pulse is to charge a capacitor
• The capacitor serves as a storage device for a signal
• For peak detection, the capacitor is charged from a circuit that allows only a build up of signal (a diode achieves this) and the final or peak signal represents the maximum signal obtained
• In an integrator the charge on the capacitor represents the signal integral between the reset and the hold signals
• A pulse width is collected by charging a capacitor from the output of a linear ramp generator which starts at a preset time and ends when a signal reached a predetermined minimum – the voltage stored in the capacitor is proportional to the duration (time length) of the pulse
Trigger signals• The purpose of a trigger signal is to be sure that the measurement
made is made on an appropriate signal not noise or an unwanted signal
• Frequently we use larger rather than smaller signal – example include light scatter or fluorescence
• The key component is called the comparator circuit which is designed around an analog and a digital input.
• The circuit is designed to have a constant voltage (set by the operator) and a signal from the sample – by comparing the preset signal with the sample signal, a cell is collected if it meets the criterion (digital signal is 1, or rejected (digital signal is 0).
• Noise of no cells passing through the observation point means that the comparator circuit output is a logical - so no signal is collected or passed onto the rest of the detection circuit (this means the computer does not have to waste valuable time)
• Coincidence occurs if a second cells arrives before the circuit has been reset – this could mean both cells are aborted, however with more sophisticated electronics, all the signals can be collected in “pipeline” which can be interrogated to resolve the conflict without losing either signal