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I asked this question in Physics stack exchange and had an answer, but I couldn't understand it well. A comment said this stack exchange was a better fit for the question.

Question:

How do I connect (which wire to which terminal) a four way key to a potentiometer, to compare the EMF of two cells. Circuit diagram and a four way key are shown in the pictures below.

A vague 3d representation of the whole circuit will be very much appreciated. I need to know where I have to connect the potentiometer and the cell and where do I connect the galvanometer?

four way key

schematic diagram

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    \$\begingroup\$ What is a four-way key? You show a picture, but I've never seen one of those before and I can't seem to work out what it's meant to do. Your schematic is also very odd. What are K, HR, and G? What's J? Why the long winding wire? \$\endgroup\$
    – Hearth
    Commented Feb 14, 2019 at 15:41
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    \$\begingroup\$ I hate trying to interpret schematics drawn by physicists ... \$\endgroup\$
    – brhans
    Commented Feb 14, 2019 at 15:43
  • \$\begingroup\$ It's a kind of switch. It has four curved metal segments, which can be joined electrically by pushing a tapered plug into the gap. These are seen in physics labs, because the contact resistance is (or at least can be) very low, minimizing experimental error. Also, since all pieces are the same metal (usually brass), there are no galvanic offsets. However, it isn't at all clear how the 4-way key applies to the given schematic. \$\endgroup\$
    – Dave Tweed
    Commented Feb 14, 2019 at 15:55
  • \$\begingroup\$ I suggest you grab a standard ohm meter and try to reverse engineer the diagram using some measured values. My guess is that the six black dots, near the E1, E2 text, are forming the four contact pairs. \$\endgroup\$
    – Oldfart
    Commented Feb 14, 2019 at 16:02
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    \$\begingroup\$ @Febin.K.Dominic Physicists draw weird schematics. Rh is the symbol for a potentiometer, not a rheostat. \$\endgroup\$
    – Hearth
    Commented Feb 15, 2019 at 15:47

2 Answers 2

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If those two cells are truly isolated, then a simpler circuit is possible. You don't need both changeover switches to select between their EMF's, only one is necessary, and that can be implemented with your 4-way key.

schematic

simulate this circuit – Schematic created using CircuitLab

Close SW1 to select V3, or SW2 to select V4

If you do want to switch the positive terminals as well, then you need a six-way key, or two of the four-way ones.

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Thanks to @KV18 for answering my question on Physics stackexchange This is his answer (and it turned out to be right. I Asked the teacher.)

Make the connections as below. Here the dots are the connection points and cross-marks are the key knobs.

enter image description here

When you want to measure the deflections on the galvanometer produced by E1, remove the middle knob of the path along E2. By removing a key knob, you are leaving it in an open circuit, and hence no current passes through. If you want to measure the deflections due to E2, then remove the middle knob in between

The output of E here means the connection between the switch and the galvanometer. As to the potentiometer (assuming you are using a wooden board with the wires arranged as you have shown in the diagram), the wire is attached to the input port (a metal port at one end of the wire). Tie the rest of the wires around that same port - I.e. from the other batteries you’re comparing the end for (or electrolytes).

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