Using Oscilloscope So That Nothing Blows Up
@Tom Carpenter's comment is correct. All oscilloscopes are "mains-Earth" grounded. Which means that the ground lead is connected to the ground pin of it's power plug. So what you actually did was this:



The ground lead always must be connected to the actual ground of the circuit under test when using an oscilloscope.*

*Unless you use an isolation transformer on the device under test, with NO chassis or Earth ground.

Also what Tom means by "making the oscilloscope do subtraction" is to connect two channels across the resistor, and grounds to actual ground. On the display will be two waveforms. Enter the "math" function of the oscilloscope, and instruct it to subtract channel 2 from channel 1. The result will be the difference between the two, which is the voltage across the resistor.