Hi Alex,
The existing RadialScale control does not have IsLogarithmic property at all. If you need gauge with logarithmic scale then you should use the Distribution property of the RadialScale. You can find more details here:
http://www.telerik.com/help/silverlight/radgauge-features-linear-and-radial-scales-basics.html#Logarithmic_Scale
For example:
Regards,
Andrey Murzov
Telerik