First time poster. I love the Bartimaeus sequence (my favourite books) and was curious about a few things.
1. Why are some magicians more powerful than others, what qualities made Gladstone so potent?
2. Is there a limit to the number of spirits a magician could summon, it seems that summons are physically demanding to do so but is maintaining the summons also draining?
3. In Ptolemy's gate when Kitty first summoned Bartimaeus and mentions Ptolemy, Ptolemy seems to possess Bartimaeus and perform magic that Bartimaeus shouldn’t be able to do in his weakened and bound state, how is that possible?

Magicians' powers seem to be linked to their strength of will and memory for reciting long summonings. Gladstone probably had both of those on his side, as well as a strong desire for power.

You need to be technically competent to be a powerful magician. I wonder if power is dependant on how proactive you are?
Also do you think that Nathaniel would have become as powerful as Gladstone given time to reach full maturity?

1. I think its because they're human and some humans are stronger than others. For their skill, magicians rely on their mental capacity, their physical strength and their emotional strength too. So magicians who are smarter, more intelligent, and have sharper memories will be able to perform more powerful summons than magicians who just aren't that smart. This is why Nathaniel, as a teen, was able to summon Bartimaeus, a djinn that Arthur would have never been able to control, despite being older and more experienced. Magicians are also affected by their strength and grow weaker as they age or become unwell.

2. Yes there is a limit and it is related to the strengths I've mentioned above. In The Ring of Solomon, we're introduced to Khaba, who is able to control at least 8 high-level djinn, one marid, a handful of foliots, and an entity that might be an afrit. So clearly he's a very powerful magician who can control a lot of different powerful entities because he has the strength to do so. On the other hand, Bartimaeus mentions that most magicians are only strong enough to summon and control a handful of foliots. In the same book, Bartimaeus mentions that with time, the strain of the summons grows and sometimes, djinn look for the magician to lose their strength and then lose control so that they can break free from their bond. So yea, it is draining for magicians to maintain their summons especially when dealing with higher level entities.

3. I don't really understand this bit, I would have to read the book again to answer you

1. I think it's one part talent, one part willpower and one part wit and learning.

2. Yeah, totally. Just as Drewdru said... But magicians can widen their possibilities by using artifacts like Solomo's ring or Gladstone's staff. Such things make the whole summoning much faster and easier, but you have to be already pretty strong to be able to use 'em. Nathaniel was strong, that's why he could activate it, but he was still far from Gladstone, and couldn't control it until Bartimaeus helped him out in Ptolemy's Gate.
I don't think Nathaniel could ever be as strong as Gladstone... He had the talent and the brains but he wasn't unscrupulous and ruthless enough.

3. I don't think we could call that a possession... It was just Bartimaeus getting really pissed off. At least we know he has some pressure points too... At the end nothing changed in the room, so I guess it was only an illusion, and it might have seen very impressive and scary for Kitty, but I don't think it would take too much strength to create it, even if Bartimaeus was weakened at the time. And even though he can only move between the limits of the circle, he can create illusions on the outside too. The first time when Nathaniel was allowed to watch Underwood summon a demon, it stayed invisible and created an illusion that made Nathaniel see the candles fall and set a bunch of papers on fire, so he almost stepped out of his circle. Therefore I don't think Bartimaeus could've actually hurt Kitty there, he could only scare her half dead and hope that she makes a mistake.