Electromagnet coil core to yoke ratio?

[britishink]

I'm building pairs of coils with standard core heights and diameters and linking each pair with a yoke. The magnets all run at 6V DC with windings of 24 AWG magnet wire in 6/8/10 layers.

Is there a ratio of coil core mass to yoke mass for maximum efficiency? (shown below)

 

[Bob S]

The cross section of the yoke should be roughly at least twice the cross section of the 5/16" diameter soft iron core. More iron is not needed.

The current rating of the 24 AWG wire is about 0.6 amps. The average power in each coil should not exceed about 2 watts (it should not be too hot to touch). The wire resistance is about 26 ohms per 1000 feet. So calculate the length of the wire and the I-squared R losses. Calculate also the max current with 6 volts dc.

If you plan to use the magnet as pictured, you will need about ~40,000 amp-turns per coil to get ~1 Tesla in the air gap. If you plan the close the magnet with another yoke on the top, then you will need about ~30 amp-turns per coil to get ~1 Tesla inside the iron (no gap).

Bob S