A pair of electrodes are placed in a solution, and the resistance is read. The reciprocal of the resistance in ohms is conductance in siemens. That divided by the distance between the electrodes is the siemens/cm, that time 10^6 is the microsiemens/cm.
I glanced at
http://books.google.com/books?id=6pSXSvGwOqAC&pg=PA1318&lpg=PA1318&dq=%22siemens+centimeter%22&source=web&ots=DgYfYa1Zg1&sig=09MMrT5HWiwAkYdh81r6Cg4aRyA#PPA1328,M1
A pair of electrodes are placed in a solution, and the resistance is read. The reciprocal of the resistance in ohms is conductance in siemens. That divided by the distance between the electrodes is the siemens/cm, that time 10^6 is the microsiemens/cm.