Miller,
In simple terms that make sense, one can think of the conductivity switch as doing the following; the conductivity switch basically does nothing but shut off one of the tones when selected off the ALL position. So, if the conductivity switch is switched to HI conductivity mode, the high tones are shut off and only the low tones are heard. If LOW conductivity is selected then the low tones are shut off and only the high tones are allowed through.
Whether an object changes tones and just where it changes is determined in simple terms by how the object relates to the ground signal. The ground balance control is designed such that a secondary signal is amplified and subtracted from the main signal. When this secondary signal from the ground equals the main signal, the ground response is essentially eliminated and no response or minimal response is heard.
Now, the ground is nothing more than another target. The ground balance control is nothing more than the adjustment of the secondary signal that provides the necessary range of amplification to allow for this cancellation to occur. The range of this ground balance adjustment is such that certain other objects can be cancelled just like the ground signal. There are many objects than fall into this category and can be cancelled but there are also other objects that can't be cancelled.
A typical silver coin greater than a dime is one object that won't allow cancellation with the GB control. A small gold nugget is another object that will not be cancelled completely by adjusting the GB control through the full range.
What happens is, when this secondary signal is amplified it may reach a point on something such as the ground such that such that the secondary signal is to equal the main signal, and this allows for the cancellation. If that same secondary signal is amplified more, the secondary signal will be greater than the main signal and then will be the dominant response. If this happens, the signal becomes a low tone.
So, at a normal ground balance setting of 9 on the GB control, something such as a nail will normally respond as a low tone. Change the GB control and at some point less than 9, that nail signal will be minimized. Because of the strange response of nails a true balancing of nails is really not practical, but the signal can be minimized or at least reduced dramatically. At a setting of less than this balance point, the signal from that same nail now becomes a high tone.
Keep in mind that as the GB is altered, the signal from the ground is also altered. So, at a GB of 4, the ground will respond as a positive response. A GB setting greater than 9 will cause a ground response that is negative. This negative response may be more difficult to notice but it is there.
So, in effect, the ground signal can be a positive response, little or no response, or a negative response. Just which response the ground indicates will be determined by the GB control setting. This same condition holds true for other objects such as nails or many other objects for that matter. Since the GB settings were designed only to handle the ground, there is no calibration of other objects. So, it is up to the operator to determine how other objects respond by simply testing and documenting.
What complicates the matter is there are no absolutes, so any documentation will show that most objects that are not mirror images of each other may not respond the same. Exceptions are limited and fortunately, most US coins do display a level of consistency. So, something like a dime response will change very little. There will be some minor change when going from silver to clad but that usually isn't nearly as much of a change as the penny change that occurs when testing zinc, versus copper, versus copper Indian Head.
If you are a gold hunter, this target transition from low conductor to high conductor is far more complicated because of other factors. The surface characteristics, thickness, purity, and overall shape can cause two equal weight objects to react totally differently. In other words, a half oz nugget found in one are could respond as a low conductor while a nugget found somewhere else may signal as a high conductor at a normal GB setting of 9.
There will be a range of nuggets that will always respond as a low conductor, but a similar transition to a high conductor signal may never occur or require a very huge nugget to do so.
This same characteristic variation can occur on other objects such as buttons, etc. In the case of bullets, size becomes a factor as to where they change from a low conductor to a high conductor. Shape may also be a factor. In other words, a dropped mini ball may not respond identical to a bullet that has flattened from hitting something.
So, this change from a low conductor signal to a high conductor signal is something a person should try to determine when trying to focus on finding a particular object. Something as basic as the orientation of the object in the ground can have an influence, so one shouldn't be alarmed of a target doesn't react like one might hope or expect. This is further complicated by the fact that other objects within the field of the coil can also alter a target's response.
In other words, in simple terms, there is no way to determine just how any target may respond. This especially holds true of similar type objects made by different companies or out of different materials.
I hope this helps clear up what is happening. Any further detailed explanation would probably take a book which I really don't feel like writing at this time.
Reg