AC current probe

It was therefore determined that provided the tip is initially positioned next to a cell, and at such a distance so that it cannot collide with the cell, the distance feedback loop can be disabled, the stimulus applied, and the tip rastered at fixed height above the cell. In such a case, the AC current probes the tip position with respect to the cell, while the amperometric current reveals individual release events (Figure 17.31). Impedance feedback control of the sample-probe separation was also used to position an SECM tip above single cells and monitor amperometrically metabolic cellular consumption [105]. 

Historically, the first and most important capacitance method is the vibrating capacitor approach implemented by Lord Kelvin in 1897. In this technique (now called the Kelvin probe), the reference plate moves relative to the sample surface at some constant frequency and tlie capacitance changes as tlie interelectrode separation changes. An AC current thus flows in the external circuit. Upon reduction of the electric field to zero, the AC current is also reduced to zero. Originally, Kelvin detected the zero point manually using his quadrant electrometer. Nowadays, there are many elegant and sensitive versions of this technique. A piezoceramic foil can be used to vibrate the reference plate. To minimize noise and maximize sensitivity, a phase-locked... 

An ac and/or dc current probe for the oscilloscope. Especially needed for switching power supply design. Some appropriate models are Tektronics P6021 or P6022 and A6302 or A6303, or better. 

As indicated above, the Model 611 does not require a separate temperature probe and so it has no temperature knob to be operated its circuits instead perform the following functions (abbreviated as in the Orion specification) (1) induce ac signal across pH probe (2) measure average dc potential of probe (3) convert amplitude of ac signal to dc potential (V) (4) calculate log V (5) measure in-phase ac current through probe (6) convert current to dc potential proportional to current (/) (7) calculate log / (8) calculate log R (resistance of probe) = log V - log I (9) convert log R into signal proportional to temperature (displayed) (10) use temperature signal to correct pH, to be read. 

Resistivity measurements are also routinely made with an ac four probe technique. The wiring would follow according to Figure 1 and the measuring currents used would be in the range 0.1 - 10 mA with frequencies of 100 Hz (9). For flux creep now known to modify susceptibility and critical current measurements care must taken with ac measurements of resistivity although for the low current densities involved the effect will not likely be observed except very close to Tc (10) or in a magnetic field. 

Our samples were fabricated with a multistep process described elsewhere [10]. The samples had structure Co(20)/Cu(10)/Co(2.5), where thicknesses are in nm. To minimize dipolar coupling between the Co layers, only the Cu(10)/ Co(2.5) layers were patterned into a nanopillar with approximate dimensions 140 x 70 nm. We measured differential resistance, dV/dl, at 295 K with four-probes and lock-in detection, adding an ac current of amplitude 20 //,A at... 

The tip and the adjacent surface form a capacitor. When the distance between tip and surface is changed by vibrating the probe an AC current flows its magnitude depends on the existing potential difference. By adjusting the external bias voltage Ucomp this potential difference can be compensated, as a consequence the AC current vanishes. In most cases relative changes of the local surface potential are of interest. In order to remove any unwanted influence of the probe surface potential a material with constant surface potential is used (typically an... 

This technique based on a hot wire thermal probe with AC excitation and 3 CO lock-in detection. Since the principle and procedures of the technique have been described in details previously [51] only a brief description is given here. We consider a thermal probe (ThP) consisting of a metallic wire of length 21 and radius r immersed in a liquid sample, acting simultaneously as a heater and as a thermometer. The sample and probe thermophysical properties are the volume specific heat pc and the thermal conductivity k, with the respective subscripts (5) and (p). The wire is excited by ac current... 

While scopes are usually used as voltage measuring devices and therefore have high input impedances, there are probes available which, by clamping around a wire, measure the ac current. This can be useful in monitoring the current flowing in NMR transmitter coils. 

Resistivity can be measured using a four-point probe [26] placed directly on the concrete surface. The probe consists of four equally spaced point electrodes that are pressed on the surface (Figure 16.12). Usually a small amount of conducting liquid is applied locally to improve the contact between electrode tips and the concrete surface. The two outer point electrodes induce the measuring current (usually with an AC current at a frequency of between about 100 and 1000 Hz (avoiding the exact values 100, 150 Hz, etc., which may induce power line interference) and the two inner electrodes measure the resulting potential drop of the electric field. The measured resistance R is given by AE/AI and the resistivity of the concrete, p, is calculated as ... 

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