Employers have different opinions about receiving thank you notes. Many appreciate the gesture, but it is difficult to say whether sending a thank you note increases your chances of getting the position. Use your judgment. You may seriously consider sending a thank you note if you had a particularly good interaction with the interviewer, or if the potential employer made a special effort to accommodate you or to treat you very well in some respect. If you have interviewed with a number of lawyers at a firm, you may consider sending a note to the recruiting coordinator, or to the hiring partner. If you do send a thank you note, remember that it also will be an example of your work product so take care to make sure it is well done.
If you decide to send a thank you note, you will also have to decide whether to send a hand-written note or email. You will hear different opinions about this question as well. A hand-written note may be a little more personal, but most lawyers are very comfortable sending and receiving all types of communications by email. Again, use your judgment based on your interview.