Alternating-current measurement

Electrical Measurements. The electrical properties of polymers have much in common with mechanical properties. They can be divided into static properties equivalent to direct current properties and dynamic properties resulting from alternating current measurements. The most used parameter is the volume or bulk resistivity (ASTM-D257-75b) which is the resistance in ohms of a material 1 cm thick and 1 cm2 area. Bulk resistivity is one of only a few properties that vary nearly 1025 in typical use (materials with values above 10 ohm-cm for polystyrene to 10 5 ohm-cm for copper). 

H. B. Hanekamp, P. Bos, and O. Vittori, The Applicability of Phase-Sensitive Alternating Current Measurements in Flow-Through Detection. Anal. Chim. Acta, 131 (1981) 149. 

The alternating current measurements over the frequency interval [8 X 10 , 2 X 10 ] Hz were performed using a Solartron mod. 1286 electrochemical interface and a Solartron mod. 1250 analyzer, controlled by the SOFTCOR-AC-PIC program [69], adopting the potentiostatic configuration and applying a sinusoidal voltage of 5 mV RMS amplitude. 

Alternating current measurements were used to investigate the electromechanical properties in combined photo-crosslinked cholesteric LCEs. In a compound showing both, a cholesteric and a chiral SmC phase, the electromechanical response in the cholesteric phase was considerably higher. 

Alternating current measurements were used to investigate the piezoelectric effect in combined photocrosslinked SmC LCEs [76]. A linear relationship was obtained for the measured piezo voltage as a function of the mechanical deformation applied. In addition it was found that the measured piezo voltage is a monotonically decreasing function of temperature for a compound showing an I-SmC phase transition. 

First, who introduced the alternating current measurements and used Nyquist plots in the solid-state electrochemistry, was Bauerle [9]. As earlier as 1967, Hartung and Mobius described... 

The apphcation of an impressed alternating current on a metal specimen can generate information on the state of the surface of the specimen. The corrosion behavior of the surface of an electrode is related to the way in which that surface responds to this electrochemical circmt. The AC impedance technique involves the application of a small sinusoidal voltage across this circuit. The frequency of that alternating signal is varied. The voltage and current response of the system are measured. 

Potential Measurement under the Influence of Alternating Currents... 

Contacts with other pipelines or grounds can be localized to within a few hundred meters by pipe current measurements. Contacts with foreign pipelines or cables can also be found by measuring potential at the fittings of the other line while the protection current of the cathodically protected pipeline is switched on and off. While the potential of unconnected pipelines will assume more positive values when the protection current is switched on, the cathodic current may also enter any line in contact with the cathodically protected pipeline and thus shift its potential to more negative values. Should the contacting line not be located by this method, fault location can be attempted with direct or alternating current. 

To avoid the errors of polarisation and stray currents, special resistivity meters are employed. One form of these uses an alternating current produced from batteries by a vibrator. The effective resistance is measured by a modified Wheatstone bridge with balance indicated by a galvanometer. 

As already indicated conductimetric measurements are normally made with alternating current of frequency 103Hz, and this leads to the existence of capacitance as well as resistance in the conductivity cell. If the frequency of the current is increased further to 106 — 107 Hz, the capacitance effect becomes even more marked, and the normal conductivity meter is no longer suitable for measuring the conductance. 

In this procedure, a constant sine wave a.c. potential of a few millivolts is superimposed upon the usual d.c. potential sweep. The applied d.c. potential is measured in the usual way and these results are coupled with measurements of the alternating current. 

---