how to create new column based on multiple columns with a function

This question is following up to my questionabout linear interpolation between two data points

I built following function from it:

def inter(colA, colB):
   s = pd.Series([colA, np.nan, colB], index= [95, 100, 102.5])
   s = s.interpolate(method='index')
   retu s.iloc[1]

Now I have a data frame that looks like this:

           on95   on102.5   on105
Index
  1         5       17        20
  2         7       15        25
  3         6       16        23

I would like to create a new column df['new'] that uses the function inter with inputs of on95 and on102.5

I tried like this:

df['new'] = inter(df['on95'],df['on102.5'])

but this resulted in NaN's.

I also tried with apply(inter) but did not find a way to make it work without an error message.

Any hints how I can solve this?