Eric Foster
New member
Here is a bit of history as these plots were done by me in 1967. The object was to look at the responses of different width brass rings and see how it affected the rate of decay. It also demonstrates that the response is that of the object which previously had been subjected to a magnetic field, and the field quickly removed. This results in eddy currents being generated in the ring, which then decay according to the current path around the ring. i.e. thicker rings, the disc, and then the bar show an ever lengthening response. At first glance it may appear that all the linear curves are what is called an exponential decay, but substituting a logarithmic amplifier instead of a linear one, shows distinct differences. In the linear display plot the signal starts from a negative value of 1000 and then decays into the 0 level toward the top of the page. The ripples and spikes are random noise cause mainly by other equipment in the building and by traffic on the road outside the laboratory. All the rings, disc and bar are the same diameter, the rings and disc are the same thickness and differ only in the depth of the plane of the ring. All the transmitter switching to reduce the current to zero takes place well within the 50uS period at the start of the traces.
The characteristic of the log amplifier is such that the gain increases as the signal reduces. This has the effect of increasing the noise the further down the trace you are. Eventually the noise would swamp the signal and the plot become meaningless. Plots 1 and 2 show an almost straight line, which is typical of a single exponential. 3 and 4 are showing a slight curvature for the first 100uS showing that the heavier cross section of these rings has a faster exponential superimposed on the primary one due to what is known as diffusion of the magnetic field in and out of the target. 5 and 6 show increasing curvature due to the ever greater diffusion that takes place in larger and more conductive sections. The log plot inverts the linear one, so that the maximum signal is at the top and decays downwards.
The large spike that appears at 400uS is just an artifact from the pulse generator.
Devices used were 9908 Optical Electronics logarithmic amplifier. Tektronix scope with sampling plug-in (one of the big tube jobs that doubled as a fan heater), and a Hewlett Packard XY plotter.
I have further plots for coins and other non-ferrous items, and also nails, bits of plated steel etc but these are for another day.
I thought I would add a soil plot. I can't remember where this soil came from, but it contains superparamagnetic iron minerals, hence the response. This shows a different feature when compared to conductive object responses. The log response is still very curved and looks little different from an exponential. However, if you plot the log of the amplitude against the log of time, then you would end up with a straight line. A typical 1/t response for non-conductive iron minerals of the superparamagnetic single domain type. A good example is Australian ironstone.
Eric.
The characteristic of the log amplifier is such that the gain increases as the signal reduces. This has the effect of increasing the noise the further down the trace you are. Eventually the noise would swamp the signal and the plot become meaningless. Plots 1 and 2 show an almost straight line, which is typical of a single exponential. 3 and 4 are showing a slight curvature for the first 100uS showing that the heavier cross section of these rings has a faster exponential superimposed on the primary one due to what is known as diffusion of the magnetic field in and out of the target. 5 and 6 show increasing curvature due to the ever greater diffusion that takes place in larger and more conductive sections. The log plot inverts the linear one, so that the maximum signal is at the top and decays downwards.
The large spike that appears at 400uS is just an artifact from the pulse generator.
Devices used were 9908 Optical Electronics logarithmic amplifier. Tektronix scope with sampling plug-in (one of the big tube jobs that doubled as a fan heater), and a Hewlett Packard XY plotter.
I have further plots for coins and other non-ferrous items, and also nails, bits of plated steel etc but these are for another day.
I thought I would add a soil plot. I can't remember where this soil came from, but it contains superparamagnetic iron minerals, hence the response. This shows a different feature when compared to conductive object responses. The log response is still very curved and looks little different from an exponential. However, if you plot the log of the amplitude against the log of time, then you would end up with a straight line. A typical 1/t response for non-conductive iron minerals of the superparamagnetic single domain type. A good example is Australian ironstone.
Eric.
