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The charge received by the sample is measured in the special sample support arrangement (shown in Figure 4) as a combination of the charge linked laterally to the sample support plates and charge retained where it is deposited and sensed by an induction electrode beneath the open backed sample [6]. The arrangement is an enhancement of that used previously [7].
Fig 4: Arrangement for measurement of corona charge
3.4 Interpretation of scuff charging observations In scuff charging studies the charge transferred to the sample is measured directly in a Faraday Pail from the charge appearing on the Teflon rod. It has separately been established that the reading of the fieldmeter for a small totally isolated area of charge is 140 per nC. It has also been shown that the reading is independent of the area of this charge. The capacitance loading of a material is defined as: CL = reading without material/(reading with material) = 140 * charge (nC) / (reading with material (V)) Measurements show that for an area of charge, say, 20mm diameter at 100mm the actual local surface voltage is 11 times the value that would arise with a large plane conducting surface. The local voltage of a 20mm charge area is thus: Vlocal = 11 * 140 * charge(nC) / (CL). Values of ‘charge decay time’ are taken as the time from Vpk to Vpk/e
3.5 Interpretation of corona charging observations The corona charge deposited on the sample is measured as a combination of the ‘conduction’ and ‘induction’ charge values (see Fig 4 above). The sensitivity of ‘induction’ observations is obtained using a sample material, such as paper, where the initial charge signal is just an induction signal and this transfers to become a conduction signal as charge moves outwards. The total charge is measured to be: Qtot = Qc + 2.33 * Qi The capacitance loading is calculated as: CL = reading without material/(reading with material) = charge (nC) * 365 / reading The 365 is measured as the reading per unit of isolated charge (nC) in the plane of the sample surface. Measurements have been made to interpret the local voltage in terms of instrument readings. Instrument readings are calibrated for a full conducting surface across the plane of the test aperture. It is shown that the local potential of a 20mm diameter area of charge has a local surface voltage 1.6x the ‘reading’. Values of ‘charge decay time’ are taken as the time from Vpk to Vpk/e. This is convenient for simple comparison between materials, but hides possibly relevant behaviour shown in the full decay curves.
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