about PhysicalMedia-Class

Question

Heym,
 
is there anyone how get experiences with this class. I tryto use it, in a normal way like this:
ManagementObjectSearcher mang = new ManagementObjectSearcher("SELECT * FROM Win32_DiskDrive");
foreach (ManagementObject obj in mang.Get())
{
MessageBox.Show(Convert.ToString(obj["Description"]));
}
Here, i used the DiskDrive-Class, and all work fine, but if i try top use the PhysicalMediaClass in the equal way, the output just gives me a zero-string.
what´s mymistake?? can anyone help me???
holzmcgyver