I have gone through several links about the dot convention of mutual inductance, but nowhere could I find the reason why a dot is placed where it's placed. Is it to do with any direction of current and field or some other reason I cannot understand? Can anyone elaborate on the physics behind the dot position?

  • \$\begingroup\$ You mean how current flows into the dotted end and out the other dotted end on a transformer? \$\endgroup\$
    – DKNguyen
    May 10, 2019 at 14:43
  • \$\begingroup\$ Yes. I think that would explain my question as well. Another way of asking this would be, if i am a manufacturer who makes inductor and lets say i wound a wire into a coil. On what basis will i place the dot? \$\endgroup\$
    – jrvinayak
    May 10, 2019 at 14:46
  • 1
    \$\begingroup\$ It's a convention. The why of it is a question for historians. As far as a physical meaning, if you wind two wires together on a core, then the spot where two wires exit the core together get the dot for both wires. \$\endgroup\$
    – TimWescott
    May 10, 2019 at 14:48
  • \$\begingroup\$ The word convention inherently means that there is no rationale but something people have agreed upon. \$\endgroup\$
    – Oldfart
    May 10, 2019 at 14:57
  • \$\begingroup\$ @TimWescott if you wind two wires together won't they exit the core together on both ends? \$\endgroup\$
    – The Photon
    May 10, 2019 at 14:58

1 Answer 1


You can understand this from the Transformer page on Hyperphysics.

To summarize, the direction of the magnetic field through the core depends on the direction the primary wire is wound around the core.

Similarly, the direction of the secondary EMF depends on the direction of the magnetic field and the direction of the secondary wire is wound around the core.

In this image (from Hyperphysics), the dots would both be placed on the upper side (or both on the lower side) of the coils, but be careful to notice the direction of the windings:

enter image description here

This analysis is derived from Faraday's Law of induction


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