Jaeger's method to find the surface tension

[shalikadm]

I'm studying for the exam day after tommorow and while reading the practical on jaeger method I found the following...

P1-P2=2T/ r
r is the radius of the capillary tube
.....
advantages of this method..,
•contact angle is not needed
•can find surface tension of liquids like mercury

my concern is as following...
we replace r with R(radius of the meniscus ) of the equation P1-P2=2T/R when the contact angle is zero. So how it can say that the contact angle is not needed. and also while the contact angle of mercury is an abtuse angle how it becomes zero for the equation ?

this is so urgent and quick response is highly appreciated...
thanks in advance !

 

[haruspex]

we replace r with R(radius of the meniscus )

Not sure what is meant by that. How are you defining radius of meniscus that makes it different from the radius of the capillary?

how it can say that the contact angle is not needed

I agree. E.g. with a 90° contact angle, the surface tension would not affect the pressures.