Why is Channel 1 Connected Across CR and LR Circuits?

[momomo_mo]

Assume there is a capacitor and resistance connecting in series in a.c. Now channel 1 of the CRO acroos both the capaciotr and resistor and channel 2 across the resistor.The trace on channel 1 was taken as the p.d. across the capacitor and that on channel 2 as the current through the capacitor.
But I want to know why channel 1 is connected across both the capacitor and resistor and not only the capacitor?

Thank You

 

[marcusl]

Most oscilloscopes measure from ground to the probe tip. If you connected one probe across the resistor and the other across the capacitor, you'd ground two different points in your circuit which would give you bad results at best or melt something at worst. So you connect both grounds together at one point, at the bottom of the resistor in your case, and the probe tips to the tops of the R and C. The difference between the two channels gives pure voltage across C.