GEB

[waltr]

That makes sense with the 3Tau rule. A US quarter has a TC of about 150us so 3 times is 450us.

[Ferric Toes]

I plotted a US clad Quarter and also some Australian ironstone. I set the quarter for a starting amplitude of 3000mV at 20uS and worked from there in 20uS steps, measuring the amplitude each time with the y cursor on a linear graph. Exactly the same for the ironstone by adjusting the quantity in the chamber to get the same start amplitude. TX width was fixed at 450uS. Plotting the results on an Excel log-log graph gives the comparison between the two decaying signals. The ironstone gives the classic t^- response where the exponent for this sample was 1.268. The quarter also behaves as expected with an increasing slope at later times. Plotting further would show that it would eventually cross the ironstone plot which continues on until disappearing in the noise at about 5mV. The time at which this happens is approximately 5mS, compared to a few hundred uS for the quarter. The second plot is a log-lin graph using the same values. The quarter reaches a single exponential after 120uS.



Eric.

[green]

I plotted a US clad Quarter and also some Australian ironstone. I set the quarter for a starting amplitude of 3000mV at 20uS and worked from there in 20uS steps, measuring the amplitude each time with the y cursor on a linear graph. Exactly the same for the ironstone by adjusting the quantity in the chamber to get the same start amplitude. TX width was fixed at 450uS. Plotting the results on an Excel log-log graph gives the comparison between the two decaying signals. The ironstone gives the classic t^- response where the exponent for this sample was 1.268. The quarter also behaves as expected with an increasing slope at later times. Plotting further would show that it would eventually cross the ironstone plot which continues on until disappearing in the noise at about 5mV. The time at which this happens is approximately 5mS, compared to a few hundred uS for the quarter. The second plot is a log-lin graph using the same values. The quarter reaches a single exponential after 120uS.

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Eric.

Thanks for the log-log chart. Don't know if I'm doing it right, I get about -1.29 for the ironstone with your chart.

[Ferric Toes]

Thanks for the log-log chart. Don't know if I'm doing it right, I get about -1.29 for the ironstone with your chart.

I let Excel do the calculation for me by drawing the trendline, clicking on power law, and 'show equation'. Ironstone, soil, and rock do vary according to their iron mineral makeup. You get single domain, where all the magnetic particles are the same size (measured in nanometres), or multi-domain for a spectrum of sizes. I get exponents of 1.01 up to 1.3 for the majority of samples from various countries. For multi domain there appears to be a slight curve on a log-log scale and if I divide the plot into two or three sections, there are slightly different slopes. At late times it gets closer to t^-1, the theoretical figure. To see this clearly, I have to plot a lot more points than in the graphs I have posted.



Eric.