oscilloscope used with isolation transformer

oscilloscope connected to isolation transformer .In the circuit diagram it shows an isolation transformer connected to an lamp dimmer. They placed the ground clip of the oscilloscope to one terminal of the transformer secondary and the probe to measure the diac. Did they place the ground at the terminal to earth reference the entire circuit, or could they have placed it anyehete and gotten same effect?******

I don't fully understand your question.It would be good if you could give us a schematic/image of the circuit.Generally in oscilloscopes,the negative terminal of the oscilloscope is internally connected to the ground of the device (oscilloscope) which in turn is connected to the earth.So proper connection of the negative terminal is very important in measurement purposes.I hope this fact gives some clarity.

The oscilloscope measures the voltage betwen it's ground clip and probe regardless of the rest of the circuitry. Think of it as being like normal test meter probes, wherever you put them, the meter measures the voltage across those points. The isolation transformer is there because on most (if not all) oscilloscopes, the ground clip is directly joined to the Earth pin in the power plug. If you ran the dimmer without the isolating transformer, the incoming AC power wires would be shorted to ground through the oscilloscope and damage would occur.

AC from socket --- Dimmer circuit --- 'Scope ground clip --- scope probe cable --- chassis of scope --- power cable --- Earth on power socket ----> Bang!

If you Earth any single point after the isolating transformer no damage is caused, the remainder of the circuit 'floats' around the ground point.

Brian.

1.

You can attach the oscilloscope clips at each end of the diac, if that is your question. It is okay because you are using an isolation transformer.

If your scope does not have a much higher input impedance than neighboring resistances, it may affect the timing network.

2.

If your question is regarding which end of the secondary should be your reference point...

If you rotate the schematic slowly, there is usually an orientation that makes it easiest to grasp the circuit's behavior.

Usually you can call bottommost wire zero ground. When you take readings at the components, the readings will correlate in a way that makes the most sense.