throuogh one electrode we introduce a constant 1.5 volts, through a
second electrode we wish to measure changes in voltage and convert to
microsiemens*
*one million ohms = one microsiemen

Siemens  is unit of electrical conductance, also called Mho
Ohm is unit of electrical resistance 
http://en.wikipedia.org/wiki/Ohm

Resistance and Conductance are two related but different quantities,
 this is not a simple unit conversion.

ohm     ? = 1/S = kg m2 / (C2)  is unit of Resistance, Impedance, Reactance 
siemens S = 1/? = C2 / (kg m2) is unit of Conductance, Admittance, Susceptance
http://encyclopedia.laborlawtalk.com/Siemens_(unit)

Example
If you measured skin resistance to be two MegaOhms

Then skin condutance is .5 micro Siemens 
as  one microsiemen is 1/ million ohms
and Conductance  C = 1/ R

 It is also important not to confuse conductivity and conductance 
Conductivity or specific conductance is the measure of the material's
ability to conduct an electric current
http://www.gvsu.edu/wri/education/manual/conductivity.htm