DC Current

Yes.

Okay.. here i have doubt because the term induction...

In my point if view Always wherever the induction persists, there will be alternating current...

No.

Can you explain please.

E.g., DC with ripple.

There are two Ls in b.........

Can you speak clearly?

what is meant 'two Ls in b.........'?

It depends on the configuration. If it's a rotating coil in a magnetic field, AC is produced, and a commutator is needed if DC is required. But if it's a conductor moving in a straight line in a magnetic field (at right angles) DC is produced.

That sentence is not "false" in the sense that being a "general" statement, does not exclude the truth. Your way of reading it is out of the scope of the quy that wrote it. Getting more specific. It's refering a) to the ways a DC current can be produced and b)The current's property (that should be unidirectional for the current to be direct) Being on the same sentence, not all a) cases should logically imply b) and obviously they don't unless other facts apply, so strictly logically it is a nonsense statement. But then should everything be that strict? S.M.

The statement is correct but incomplete. Direct current can be produced by electrostatic machines like the Van der Graaf generator.

Unless I'm much mistaken, a Van der Graaf generator does not use electromagnetic induction.

It is electrostatic induction, but it makes DC, behaving like a high voltage current source. The point is that there are ways other than batteries and electromagnetic machines to make DC. Solar cells also make DC.

The statement would be correct if it were re-worded to :-

Direct Current (DC): Current with a unidirectional flow, for example, produced by storage battery or electromagnetic induction machine with commutator (dynamo).

The original post was correct, questioning the "electromagnetic induction" since most machines of that type produce AC - they only produce DC with the aid of diodes or commutators. The most important word is "unidirectional", DC does not have to be of constant magnitude.

It makes one wonder about the origins of the term "Direct Current" - perhaps it was once "directed current" with a constant direction? The French term CC does not help, since DC does not have to be "Constant".

67model

But the original post referred to electromagnetic induction, that's the point I was making.

Agreed practical machines produce AC and need diodes or commutators to give DC output. But devices producing DC direct (pun intended) can be envisaged. Take 2 parallel wires with a magnetic field at right angles to the plane of the wires. If a conductor slides along the wires a constant voltage is produced between the wires.

The homopolar generator (based on Faraday Disc) does, in a practical way, what you wrote. It is good for very high DC currents at low voltage, but not competitive for normal generation.

High voltage, high power, DC generators were made in 1970s (magneto-hydrodynamic), by blowing hot conductive gas between electromagnets, with pick-up electrodes placed to take off the current. It cuts out the mechanics between hot gas and electricity. But the materials to "seed" the gas, to make it conductive, were corrosive and difficult to re-cycle and the system was not competitive, despite intensive research.

I objected to the definition quoted by the O.P., because it suggests that only unidirectional current made by the methods it specified can be called "DC".

67model

Yes, good point, I'd forgotten about the disc generator.