Here is a stupid question: If you always have to connect the tip to ground, as your (-) at the scope side is connected to ground, why do you need the tip at all, as you are always measuring the potential difference between your signal and ground, and the scope is connected to ground already?
I actually have seen once someone measuring with a scope, and he did not connect the tip. I found it rather funny, and was pointing out that he is measuring a potential difference, and therefore needs to connect both. But from the conclusions in this posting, my view on reality was wrong, as the scope seems to measure always referenced to ground. So now I am actually in doubt myself, and believe that the guy doing this measurement without tip, might not have been an idiot after all, or do I miss out something here =)
As
tautech has pointed out,you have your terminology backwards.
The "tip" is the end of the probe which you attach to the point you wish to look at the voltage on,the "clip",or more correctly "earth clip"is the thing you attach to the "common" point of the circuit.
Most equipment contains internal power supplies.
Let us consider a device with +12v ,-12v,+5v,-5v supplies,all with one side of the supply connected to the common "rail".
This "common rail"
is connected to the chassis frame/cabinet .Forgetting the 'scope for a second,if you take a DMM,place its black lead on the chassis,& touch the red lead to the test points for the above supplies in succession,-----you will see +12v,-12v,etc.
Let us assume that you have your 'scope sitting alongside this other device,all plugged in & ready to go.
Now take your DMM & put the black lead on the outer part of the "scope's BNC connector.
Check your voltage test points again.
You will see:-+12v,-12v,etc!
What has happened?
This device,as is common in much non-domestic Electronic equipment not only returns one side of the power supplies to the chassis,but also connects the Mains safety Earth to the chassis.
An experienced Tech,looking at the dc supplies in such a device,may set his "scope to "DC coupled" at,say,5v/div,'free run" the display,centre the line vertically on the screen,& touch the probe tip to the dc supply test points,so the horizontal line will deflect +2.4 large divisions for +12v,-2.4 for -12v,+ one large div for +5v,etc.
Ideally,he would always attach the "earth clip" to the chassis/common rail,but for a quick check it is not unusual to rely upon the common connection via the Mains Earth system.
It is normal practice when using a 'scope to measure everything with respect to the common rail,which in mains operated equipment is normally Earth/chassis.
Measuring across a circuit with each side above Earth is seldom necessary,but can be achieved either using a purpose designed differential probe,or by the following method.
Thinking back to our device with the multiple supplies,what if we wanted to measure the difference in voltage between the +12v & -12v supplies?(I mean look at the complete 24v)
OK,we place both,(or only one--it doesn't matter) our "earth clips"on the common/rail/chassis(we shouldn't rely on the Mains Earth connection here,as it may introduce errors).
Connect CH1 probe to one supply,CH2 to the other.
You will,with 5v /div on each channel,see two lines 4.8 large divisions apart.
Make one line the reference,& you will read off + or - 24 volts depending on which you chose.
Perhaps the -ve & +ve lines need to be the same numerical value--you can then set the 'scope to "A+ B",& it will display the difference in that value.
I would suggest you get a couple of "AA" batteries & have a play around in the "dc coupled" mode.