Manpower

wouldn't make a lot of sense since manpower is the measure of people willing to join the military/ able to join the military.

That's a limited definition of manpower. Diverting people to research actually makes real world sense since it is a matter of priority and what a nation applies it's manpower toward.

As the mechanic is now your definition applies. My suggestion expands the use of manpower. I am more concerned with how the mechanic would effect game play, balance, and the use of gold than any philosophical argument.

well first you can't send anyone to research since they need to have the training for it. But that all aside it would ruin the balance of the game since gold would become pointless and so this will never happen. Gold needs to be unique and useful because it's the main source of income for call of war.

That was my concern. The use of gold and effect on gold...I think an exaggerated exchange rate may fix that though...Manpower is only a bottle neck early on after that it accumulates and becomes rather irrelevant. I wanted to find a way to make it relevant later in the game.