How to ask someone (outside the department) to be on the thesis committee I have to request a faculty member outside of my department to be on my thesis committee and I was wondering how I should go about doing this. To be frank, I don't really see why someone would be willing to be on the thesis committee for someone outside their department and even field - apart from the altruism of wanting to help others in academia and all, but I assume professors are busy! Therefore, I imagine that it's hard to convince someone to do this? 

In any case, I was wondering:


  what information about myself should I aim to include in an email request to a potential thesis committee member outside my department? 


In addition, while I need to find someone to file my candidacy papers, but I have only a vague idea of what my final thesis will look like (I know the field, but not the particular question it will address.)

Some things I have come up with include:


a rough description of the subject area 
'expected' graduation date and number of years spent in graduate school so far
if they have been on the thesis committee for someone else with a similar thesis (by looking at the thesis committees of former students who worked with my advisor)
From my experience, having cross-department committee members is very common, and can actually be very beneficial to the student, as you gain access to expertise and perspective you might otherwise miss. There's usually at least some connection between the research being done and the expertise of the faculty member. To use myself as an example, my research involved behavioral neuroscience, but I had both an electrical engineering faculty with signal processing experience and a psychology faculty on my committee, and they both gave very useful advice with my project. That being said, I had a committee member who was completely unrelated to my work (my focus shifted midway through my PhD career), and he requested to still stay on despite the lack of direct research connection; he was happy just to give advice and provide feedback.

You can find potential committee members by talking to other graduate students, and by looking at what theses professors have worked on (as you suggest above). I definitely recommend talking to other students; some professors are more difficult to work with than others, and you likely won't get that type of information from your advisor.

Regarding how to go about talking to the professor, I would just send a simple email asking for a meeting about this topic. If they're simply not interested, they'll say so, and you'll save yourself time. If they are, meet to discuss the following:


who you work for
what you're researching (both the ten-year research goal, if there is one, as well as your specific project)
why you think this faculty member can help you
why this person should be interested (this should be tacit; i.e., you shouldn't have to say it explicitly, he should understand it from the above items)