Depolarisation effect

Fig. 6. Calculated optical absorption spectra of a metallic CNT in a magnetic flux. In the case that the electric field is parallel to the axis (left), the absorption exhibits a distinct AB effect. In the case of the perpendicular polarisation (right) the depolarisation effect suppresses the absorption almost completely.

Current efficiency may be increased by adding a substance such as hydrofluoric acid which raises the anode potential, and also by adding sulphurous acid or hydrogen sulphide which destroys Caro s acid but does not affect persulphuric acid. The addition of sulphurous acid to the point of saturation in sulphuric acid of density 1 38 raises the current efficiency to 92 per cent. The addition of hydrochloric acid to the bath has a beneficial effect because it raises the anode potential and also destroys Caro s add, and so removes the harmful depolarising effect of this substance. It has been shown that the concentration of persulphuric acid increases with rise in current density, but the final concentration of Caro s acid is independent of the current density. 

Compounds having structural similarity with acetylcholine (1) such as bephenium (3) or thenium (4) are reported to exert their action on helminths by competing for the acetylcholine receptor site. The difference between these compounds and acetylcholine is that the former are not biodegraded by AChE and, therefore, the depolarisation effect persists very long. The net results is the contraction of the muscle causing spastic paralysis of the worm. 

The compounds of this class may not bear any obvious similarity with the acetylcholine molecule but would exert the anthelmintic action by blocking the acetylcholine receptor site. Consequently, acetylcholine is unable to increase the permeability of the membrane to initiate a depolarisation effect. 

TSD experiments are performed using the measuring circuit shown in Figure 5.1. The experimental method is schematically drawn in Figure 5.28. The sample is heated to polarisation temperature (Tp), at time t an electrical field (B,) starts to polarise the sample. After a certain polarisation time (tpl) the temperature is decreased to T the polarisation is then frozen in. At temperature T (i.e. at time t,) the electrical field (E ) is removed and a small depolarisation current effect is measured. If this depolarisation effect has been vanished conpletely, the temperature is to increased linearly as a function of time and the thermally stimulated discharge current is recorded. 

TSD dipole relaxation processes are in general easier detectable than space charge depolarisation effects. For nonpolar polymers, however, the TSD effect is depending on the space charge polarisation possibilities. 

An example of a PVC orientation depolarisation effect, measured with a combined TMA/TSD system is given in chapter 6. These orientation depolarisation effects were measured on small (i.e. 8 mm.) diameter, samples. Such samples proved to be too small, however, to detect the space charge depolarisation effects in non-polar SSBR rubbers. These non-vulcanised rubber samples were pressed, therefore, at 140°C between two (l mm thick) brass disks with a diameter of respectively 110 mm (high potential electrode) and 80 mm (low potential electrode) to a sample thickness of about 0.2 mm. A ring (inner/outer diameter 75/85 mm) of 50 micron thick Vespel foil avoided shortcircuiting between the two brass disks. 

Except for a few special cases, there are similar polarisation effects for other angles between the optical axis of the excitation and the X-Z plane, different polarisation of the excitation, and even for unpolarised excitation. Methods to reject depolarisation effects from the measured decay curves in different geometric configurations are discussed in detail in [355, 389, 476]. 

Simple systems such as that shown in Fig. 5.11 yield an excellent optical efficiency, and are almost free of pulse dispersion and wavelength-dependent pulse shift. Another benefit is that the detection path is almost free of polarisation effects compared to monochromator-based systems. The high numerical aperture of the light eollection system further reduces the influence of the rotational relaxation see Fig. 5.9. With aspheric lenses an NA of around 1 can be achieved, and at an NA this high the polariser in the detection path can often be omitted. In the setup in the left graphic of Fig. 5.11, residual depolarisation effects can be removed by slightly tilting the polarisation direetion of the laser. 

The nicotinic receptor AChNR is quite different from the muscarinic receptor, ft is a Ugand-gated ion channel and does not operate with a second messenger system. Activation of the nicotinic receptor leads to a flow of Na and IC " across the cell membrane and a depolarising effect on the effector cell (nerve cell or neuromuscular end plate). 

Rosenfel d" considers that SO2 can act as a depolariser of the cathodic process. However, this effect has only been demonstrated with much higher levels of SO2 (0-5%) than are found in the atmosphere (Table 2.4) and the importance of this action of SO2 has yet to be proved for practical environments. However, SO2 is 1 300 times more soluble than O2 in water" and therefore its concentration in solution may be considerably greater than would be expected from partial pressure considerations. This high solubility would make it a more effective cathode reactant than dissolved oxygen even though its concentration in the atmosphere is comparatively small. 

Sulphur dioxide in the air originates from the combustion of fuel and influences rusting in a number of ways. For example, Russian workers consider that it acts as a cathodic depolariser , which is far more effective than dissolved oxygen in stimulating the corrosion rate. However, it is the series of anodic reactions culminating in the formation of ferrous sulphate that are generally considered to be of particular importance. Sulphur dioxide in the air is oxidised to sulphur trioxide, which reacts with moisture to form sulphuric acid, and this in turn reacts with the steel to form ferrous sulphate. Examination of rust Aims formed in industrial atmospheres have shown that 5% or more of the rust is present in the form of iron sulphates and FeS04 4H2 0 has been identified in shallow pits . 

Reducing agents have the same ultimate effect as cathodic depolarisation in that they convert anodic regions to cathodic and increase the ratio of cathodic to anodic areas. 

The acid concentration of the solution must not be too great, otherwise the deposition of the copper may be incomplete or the deposit will not adhere satisfactorily to the cathode. The beneficial effect of nitrate ion is due to its depolarising action at the cathode ... 

An effect of opening K+ channels is to hypetpolarise the primary sensory neurons. Similarly to local anaesthetics, this makes the cell less likely to produce an action potential because more depolarising stimuli are needed to overcome the block. NS 1619 is an example of this type of drug which has initially shown antitussive activity in a variety of experimental systems. 

Both disodium cromoglycate and nedocromil sodium have antitussive effects in humans. In this instance, their activity occurs by increasing the depolarisation of sensory nerves, which increases the threshold for an action potential and therefore inhibits the activity of these neurons. 

This effect appears to be of importance in the case of normal galvanic cells, the electromotive forces of which depend on the concentration of solutions in equilibrium with depolarising solids such as calomel or mercurous sulphate. The exact relationships are, unfortunately, not yet wholly elucidated. 

Figure 1.6 Presynaptic inhibition of the form seen in the dorsal horn of the spinal cord, (a) The axon terminal (i) of a local neuron is shown making an axo-axonal contact with a primary afferent excitatory input (ii). (b) A schematic enlargement of the synapse, (c) Depolarisation of the afferent terminal (ii) at its normal resting potential by an arriving action potential leads to the optimal release of neurotransmitter, (d) When the afferent terminal (ii) is already partially depolarised by the neurotransmitter released onto it by (i) the arriving acting potential releases less transmitter and so the input is less effective...

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