I am wanting to use an electro magnet much like this to hold a 60mm diameter steel spere for use in a test rig.

http://uk.rs-online.com/web/p/holding-magnets-solenoid-accessories/0346081/

I previously bought another magnet that was rated to carry 2.5 kg.
The steel sphere weight approx 1.1kg, but because of it being spherical and not fully contacting the face of the magnet, it was barely able to hold it.

So my thinking is to use a larger magnet with a greater holding force to overcome the inefficient contact area.  
However a largewr diameter magnet means that the outer case of will be further from the sphere, only the inner core will be in contact.  Is this ok or will it cause me problems?  

Thanks.   

Why not make a cup shaped pole piece to increase the contact with the magnet you have?

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.
 

That is the key, a cup shaped pole piece.
You could just about any pole piece configuration as long as you machined the surface with the spherical contact shape.

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