Sign of material's thermopower

Hi,

I am abit confused regarding how to determine the sign of the seebeck coefficient.

S= [delta V]/ [delta T]

where both values are Hot - Cold

Is this the right defination?, there are also claims that there should be a negative sign in front of it which is kind of confusing.

And normally when you measure the thermopower of a material with respect to a material let say its copper, do you minus or plus to get the absolute thermoelectric power? I've read various journal which minus and plus.

regards,

Edwin