[Python] Create column from another dataframe, using existing columns

I have a pandas DataFrame with columns "Quantity" and "Unit", e.g.:

Quantity Unit
0 Current A
1 Current mA
2 Voltage V
3 Length m
4 Length km


and I have a pandas DataFrame with ratios of units, e.g.:

Current Voltage Length
A 1 NaN NaN
mA 1000 NaN NaN
V NaN 1 NaN
m NaN NaN 1000
km NaN NaN 1


Is it possible to create in first dataframe column with values of ratios?:

Quantity Unit Ratio
0 Current A 1
1 Current mA 1000
2 Voltage V 1
3 Length m 1
4 Length km 1000


I have tried to get every value by iterrows, but I have really big dataframe and it's not working correctly in case of time. Maybe there is another, more expedient way to solve my issue, but I have to work with dataframes of ratios.

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